perf annotation: Add symbol__get_annotation

Add a new utility function to get an function annotation out of existing
code.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1432749114-904-4-git-send-email-andi@firstfloor.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>