perf probe: Fix build when DWARF support libraries not present

perf probe: Fix build when DWARF support libraries not present

On a freshly installed system, after libelf-dev is installed we get:

    CC       /tmp/build/perf/util/probe-event.o
  util/probe-event.c: In function ‘try_to_find_probe_trace_events’:
  util/probe-event.c:753:46: error: unused parameter ‘target’ [-Werror=unused-parameter]
       int max_tevs __maybe_unused, const char *target)
                                                ^
    CC       /tmp/build/perf/util/cgroup.o
  util/probe-event.c: At top level:
  util/probe-event.c:193:12: error: ‘get_text_start_address’ defined but not used [-Werror=unused-function]
   static int get_text_start_address(const char *exec, unsigned long *address)
            ^
  cc1: all warnings being treated as errors
  make[1]: *** [/tmp/build/perf/util/probe-event.o] Error 1
  make[1]: *** Waiting for unfinished jobs....
  make: *** [install] Error 2

Fix it by enclosing functions only used when those libraries are installed
under the suitable preprocessor define and using __maybe_unused to a function
that is only built when DWARF support is disabled.

Problem introduced in this changeset:

  commit fb7345bbf7fad9bf72ef63a19c707970b9685812
  Author: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
  Date:   Thu Dec 26 05:41:53 2013 +0000

      perf probe: Support basic dwarf-based operations on uprobe events

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-73kc2fopt81517hrdgdra18o@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>