libtraceevent: Fix build warning on 32-bit arches

Fixed a compilation warning for casting to pointer from integer of
different size on 32-bit platforms.

Reported-by: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: linux-trace-devel@vger.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>