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 <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Link: http://lkml.kernel.org/r/20170212024655.GA15997@udknight
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

tools/perf/util/scripting-engines/Build

@@ -1,6 +1,6 @@
 libperf-$(CONFIG_LIBPERL)   += trace-event-perl.o
 libperf-$(CONFIG_LIBPYTHON) += trace-event-python.o
 
-CFLAGS_trace-event-perl.o += $(PERL_EMBED_CCOPTS) -Wno-redundant-decls -Wno-strict-prototypes -Wno-unused-parameter -Wno-shadow -Wno-undef -Wno-switch-default
+CFLAGS_trace-event-perl.o += $(PERL_EMBED_CCOPTS) -Wno-redundant-decls -Wno-strict-prototypes -Wno-unused-parameter -Wno-shadow -Wno-nested-externs -Wno-undef -Wno-switch-default
 
 CFLAGS_trace-event-python.o += $(PYTHON_EMBED_CCOPTS) -Wno-redundant-decls -Wno-strict-prototypes -Wno-unused-parameter -Wno-shadow