tools lib traceevent: Copy trace_clock and free it

tools lib traceevent: Copy trace_clock and free it

The pevent->trace_clock should not be a direct pointer to what was
given. It should be copied and freed.

Note, valgrind pointed this out when a caller passed in a pointer that
needed to be freed and it never was. Ideally, pevent should copy it
(which this change does), and free the copy. It's up to the caller to
free the clock string passed in.

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/20150324135922.695906738@goodmis.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>