tools lib traceevent: Free filter tokens in process_filter()

valgrind showed that the filter token wasn't being freed properly in
process_filter().

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20150324135923.817723903@goodmis.org


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