tracing/events: fix lockdep system name

Impact: fix compile error of lockdep event tracer

Ingo Molnar pointed out that the system name for the lockdep tracer was "lock"
which is used to include the event trace file name. It should be "lockdep"

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>