perf tools: Fix old GCC build error in trace-event-parse.c:parse_proc_kallsyms()

perf tools: Fix old GCC build error in trace-event-parse.c:parse_proc_kallsyms()

Old GCC (4.1) does not see through the code flow of parse_proc_kallsyms()
and gets confused about the status of 'fmt':

 util/trace-event-parse.c: In function ‘parse_proc_kallsyms’:
 util/trace-event-parse.c:189: warning: ‘fmt’ may be used uninitialized in this function
 make: *** [util/trace-event-parse.o] Error 1

Help out GCC by initializing 'fmt' to NULL.

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20130912131649.GC23826@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>