perf scripting perl: Fix compile error with some perl5 versions

perf scripting perl: Fix compile error with some perl5 versions

Fix below compile error:

  CC       util/scripting-engines/trace-event-perl.o
  In file included from /usr/lib/perl5/5.22.2/i686-linux/CORE/perl.h:5673:0,
                   from util/scripting-engines/trace-event-perl.c:31:
  /usr/lib/perl5/5.22.2/i686-linux/CORE/inline.h: In function 'S__is_utf8_char_slow':
  /usr/lib/perl5/5.22.2/i686-linux/CORE/inline.h:270:5: error: nested extern declaration of 'Perl___notused' [-Werror=nested-externs]
          dTHX;   /* The function called below requires thread context */
			     ^
  cc1: all warnings being treated as errors

After digging perl5 repository, I find out that we will meet this
compile error with perl from v5.21.1 to v5.25.4

Signed-off-by: Wang YanQing <udknight@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20170212024655.GA15997@udknight
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>