perf symbols: Define STT_GNU_IFUNC for glibc 2.9 and older.

The token STT_GNU_IFUNC is not available with glibc 2.9 and older.
Define this token if it is not already defined.

This patch fixes this build errors with older versions of glibc.

  CC       util/symbol-elf.o
util/symbol-elf.c: In function ‘elf_sym__is_function’:
util/symbol-elf.c:75: error: ‘STT_GNU_IFUNC’ undeclared (first use in this function)
util/symbol-elf.c:75: error: (Each undeclared identifier is reported only once
util/symbol-elf.c:75: error: for each function it appears in.)
make: *** [util/symbol-elf.o] Error 1

Signed-off-by: Vinson Lee <vlee@twitter.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Anton Blanchard <anton@samba.org>
Cc: Avi Kivity <avi@cloudius-systems.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Cc: Waiman Long <Waiman.Long@hp.com>
Cc: stable@vger.kernel.org # 3.17+
Link: http://lkml.kernel.org/r/1423528286-13630-1-git-send-email-vlee@twopensource.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>