perf/live: don't synthesize build ids at the end of a live mode trace

It doesn't really make sense to record the build ids at the end of a
live mode session - live mode samples need that information during the
trace rather than at the end.

Leave event__synthesize_build_id() in place, however; we'll still be
using that to synthesize build ids in a more timely fashion in a
future patch.

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <1272696080-16435-2-git-send-email-tzanussi@gmail.com>
Signed-off-by: Tom Zanussi <tzanussi@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>