perf evlist: Export id_add_fd()

Will be used to storing the event IDs in evlist object so it get stored
into perf.data file.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Kan Liang <kan.liang@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1446734469-11352-6-git-send-email-jolsa@kernel.org


[ Split from the patch storing the ids in the perf.data file ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>