tools lib traceevent: It's preempt not prempt

Fix the typo of the function name pevent_data_prempt_count()

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Fixes: c52d9e4e ("tools lib traceevent: Add retrieval of preempt count and latency flags")
Link: http://lkml.kernel.org/r/20170216201352.469c99de@grimm.local.home


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