perf python: Use -Wno-redundant-decls to build with PYTHON=python3

perf python: Use -Wno-redundant-decls to build with PYTHON=python3

When building in ClearLinux using 'make PYTHON=python3' with gcc 8.2.1
it fails with:

    GEN      /tmp/build/perf/python/perf.so
  In file included from /usr/include/python3.7m/Python.h:126,
                   from /git/linux/tools/perf/util/python.c:2:
  /usr/include/python3.7m/import.h:58:24: error: redundant redeclaration of ‘_PyImport_AddModuleObject’ [-Werror=redundant-decls]
   PyAPI_FUNC(PyObject *) _PyImport_AddModuleObject(PyObject *, PyObject *);
                          ^~~~~~~~~~~~~~~~~~~~~~~~~
  /usr/include/python3.7m/import.h:47:24: note: previous declaration of ‘_PyImport_AddModuleObject’ was here
   PyAPI_FUNC(PyObject *) _PyImport_AddModuleObject(PyObject *name,
                          ^~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors
  error: command 'gcc' failed with exit status 1

And indeed there is a redundant declaration in that Python.h file, one
with parameter names and the other without, so just add
-Wno-error=redundant-decls to the python setup instructions.

Now perf builds with gcc in ClearLinux with the following Dockerfile:

  # docker.io/acmel/linux-perf-tools-build-clearlinux:latest
  FROM docker.io/clearlinux:latest
  MAINTAINER Arnaldo Carvalho de Melo <acme@kernel.org>
  RUN swupd update && \
      swupd bundle-add sysadmin-basic-dev
  RUN mkdir -m 777 -p /git /tmp/build/perf /tmp/build/objtool /tmp/build/linux && \
      groupadd -r perfbuilder && \
      useradd -m -r -g perfbuilder perfbuilder && \
      chown -R perfbuilder.perfbuilder /tmp/build/ /git/
  USER perfbuilder
  COPY rx_and_build.sh /
  ENV EXTRA_MAKE_ARGS=PYTHON=python3
  ENTRYPOINT ["/rx_and_build.sh"]

Now to figure out why the build fails with clang, that is present in the
above container as detected by the rx_and_build.sh script:

  clang version 6.0.1 (tags/RELEASE_601/final)
  Target: x86_64-unknown-linux-gnu
  Thread model: posix
  InstalledDir: /usr/sbin
  make: Entering directory '/git/linux/tools/perf'
    BUILD:   Doing 'make -j4' parallel build
    HOSTCC   /tmp/build/perf/fixdep.o
    HOSTLD   /tmp/build/perf/fixdep-in.o
    LINK     /tmp/build/perf/fixdep

  Auto-detecting system features:
  ...                         dwarf: [ OFF ]
  ...            dwarf_getlocations: [ OFF ]
  ...                         glibc: [ OFF ]
  ...                          gtk2: [ OFF ]
  ...                      libaudit: [ OFF ]
  ...                        libbfd: [ OFF ]
  ...                        libelf: [ OFF ]
  ...                       libnuma: [ OFF ]
  ...        numa_num_possible_cpus: [ OFF ]
  ...                       libperl: [ OFF ]
  ...                     libpython: [ OFF ]
  ...                      libslang: [ OFF ]
  ...                     libcrypto: [ OFF ]
  ...                     libunwind: [ OFF ]
  ...            libdw-dwarf-unwind: [ OFF ]
  ...                          zlib: [ OFF ]
  ...                          lzma: [ OFF ]
  ...                     get_cpuid: [ OFF ]
  ...                           bpf: [ OFF ]

  Makefile.config:331: *** No gnu/libc-version.h found, please install glibc-dev[el].  Stop.
  make[1]: *** [Makefile.perf:206: sub-make] Error 2
  make: *** [Makefile:70: all] Error 2
  make: Leaving directory '/git/linux/tools/perf'

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thiago Macieira <thiago.macieira@intel.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-c3khb9ac86s00qxzjrueomme@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>