perf tools: Fix annotation with kcore

Patch "perf tools: Fix build-id matching on vmlinux" breaks annotation
with kcore.  The problem is that symbol__annotate() first gets the
filename based on the build-id which was previously not set.

This patch provides a quick fix, however there should probably be only
one way to determine the filename. e.g.  symbol__annotate() should use
the same way as dso__data_fd().

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1415700294-30816-1-git-send-email-adrian.hunter@intel.com


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