[libjack] fix warning about volatile return type, by typedef

the previous commit was not completely correct, and resulted in a warning.
this makes the volatile keyword apply to the function pointer and not
the void return value.

git-svn-id: svn+ssh://jackaudio.org/trunk/jack@4198 0c269be4-1314-0410-8aa9-9f06e86f4224

libjack/thread.c

@@ -111,7 +111,8 @@ jack_thread_touch_stack()
 	}
 }
 
-static volatile void (* ptr_jack_thread_touch_stack )() = jack_thread_touch_stack;
+typedef void (* stack_touch_t)();
+static volatile stack_touch_t ptr_jack_thread_touch_stack = jack_thread_touch_stack;
 
 static void*
 jack_thread_proxy (void* varg)