powerpc/ftrace: Fix obsolete comment

CONFIG_MCOUNT is not defined anymore, the corresponding #ifdef there
is CONFIG_FUNCTION_TRACER.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/entry_32.S

@@ -1463,4 +1463,4 @@ _GLOBAL(return_to_handler)
 	blr
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 
-#endif /* CONFIG_MCOUNT */
+#endif /* CONFIG_FUNCTION_TRACER */