perf tools: Fix build failure on perl script context

On my Archlinux machine, perf faild to build like below:

    CC       scripts/perl/Perf-Trace-Util/Context.o
  In file included from /usr/lib/perl5/core/perl/CORE/perl.h:3905:0,
                   from Context.xs:23:
  /usr/lib/perl5/core/perl/CORE/inline.h: In function :
  /usr/lib/perl5/core/perl/CORE/cop.h:612:13: warning: declaration of 'av'
                                  shadows a previous local [-Werror-shadow]
             AV *av =3D GvAV(PL_defgv);
                 ^
  /usr/lib/perl5/core/perl/CORE/inline.h:526:5: note: in expansion of
                                  macro 'CX_POP_SAVEARRAY'
         CX_POP_SAVEARRAY(cx);
         ^~~~~~~~~~~~~~~~
  In file included from /usr/lib/perl5/core/perl/CORE/perl.h:5853:0,
                   from Context.xs:23:
  /usr/lib/perl5/core/perl/CORE/inline.h:518:9: note:
                                  shadowed declaration is here
         AV *av;
             ^~

What I did to fix is adding '-Wno-shadow' as the error message said it's
the cause of the failure.  Since it's from the perl (not perf) code
base, we don't have the control so I just wanted to ignore the warning
when compiling perl scripting code.

Committer note:

This also fixes the build on Fedora Rawhide.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20160802024317.31725-1-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>