kernel-shark-qt: Fix the returned error value of kshark_get_event_id_easy()

This patch fixes a simple bug in kshark_get_event_id_easy().
All returned error codes must be negative.

Signed-off-by: Yordan Karadzhov <ykaradzhov@vmware.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>