perf report: Annotate variable initialization

Certain versions of GCC dont see the initialization that is done here:

  builtin-report.c: In function ‘__cmd_report’:
  builtin-report.c:1038: warning: ‘syms’ may be used uninitialized in this function

So annotate it with a NULL initialization.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>