tracing/function-graph-tracer: enabled by default

CONFIG_FUNCTION_GRAPH_TRACER depends on FUNCTION_TRACER already,
(turning it non-default) so it so making it default-n is pointless.

So enable it by default - it's a nice extension of the function tracer.
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/Kconfig

@@ -67,6 +67,7 @@ config FUNCTION_GRAPH_TRACER
 	bool "Kernel Function Graph Tracer"
 	depends on HAVE_FUNCTION_GRAPH_TRACER
 	depends on FUNCTION_TRACER
+	default y
 	help
 	  Enable the kernel to trace a function at both its return
 	  and its entry.