tracing, rcu: Hide trace event rcu_nocb_wake when not used

The trace event rcu_nocb_wake is only used when CONFIG_RCU_NOCB_CPU is
defined. But the trace event is defined regardless. As defined trace
events take up memory, it is a waste to have it defined when not used.
Surround the trace event with an #ifdef to have it only defined when it
is used.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/trace/events/rcu.h

@@ -243,6 +243,7 @@ TRACE_EVENT(rcu_exp_funnel_lock,
 		  __entry->grphi, __entry->gpevent)
 );
 
+#ifdef CONFIG_RCU_NOCB_CPU
 /*
  * Tracepoint for RCU no-CBs CPU callback handoffs.  This event is intended
  * to assist debugging of these handoffs.
@@ -285,6 +286,7 @@ TRACE_EVENT(rcu_nocb_wake,
 
 	TP_printk("%s %d %s", __entry->rcuname, __entry->cpu, __entry->reason)
 );
+#endif
 
 /*
  * Tracepoint for tasks blocking within preemptible-RCU read-side