perf bpf: Remove bpf/ subdir from bpf.h headers used to build bpf events

perf bpf: Remove bpf/ subdir from bpf.h headers used to build bpf events

The bpf.h file needed gets installed in /usr/lib/include/perf/bpf/bpf.h,
and /usr/lib/include/perf/ is added to the include path passed to clang
to build the eBPF bytecode, so just remove "bpf/", its directly in the
path passed already. This was working by accident, fix it.

I.e. now this is back working:

  # cat /home/acme/git/perf/tools/perf/examples/bpf/hello.c
  #include <stdio.h>

  int syscall_enter(openat)(void *args)
  {
  	puts("Hello, world\n");
  	return 0;
  }

  license(GPL);
  # perf trace -e /home/acme/git/perf/tools/perf/examples/bpf/hello.c
       0.000 pickup/21493 __bpf_stdout__(Hello, world)
      56.462 sh/13539 __bpf_stdout__(Hello, world)
      56.536 sh/13539 __bpf_stdout__(Hello, world)
      56.673 sh/13539 __bpf_stdout__(Hello, world)
      56.781 sh/13539 __bpf_stdout__(Hello, world)
      56.707 perf/13182 __bpf_stdout__(Hello, world)
      56.849 perf/13182 __bpf_stdout__(Hello, world)
  ^C
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-d9myswhgo8gfi3vmehdqpxa7@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>