server: use void* instead of function pointer for wl_object.implementation

This is needed to make C++ programs that include wayland-server.h build:
C++ does not allow conversions from data pointers to function pointers.

src/wayland-util.h

@@ -56,7 +56,7 @@ struct wl_interface {
 
 struct wl_object {
 	const struct wl_interface *interface;
-	void (* const * implementation)(void);
+	const void *implementation;
 	uint32_t id;
 };