tracing: current tip/master can't enable ftrace

After commit 40ada30f96,
"make menuconfig" doesn't display "Tracer" item.

Following modification restores it.

kernel/trace/Kconfig

@@ -61,6 +61,7 @@ config TRACING_SUPPORT
 	bool
 	depends on TRACE_IRQFLAGS_SUPPORT
 	depends on STACKTRACE_SUPPORT
+	default y
 
 if TRACING_SUPPORT