wlr_touch: Declare wlr_touch_impl usage as const

This brings it in line with wlr_keyboard and wlr_pointer

include/wlr/interfaces/wlr_touch.h

@@ -16,7 +16,7 @@ struct wlr_touch_impl {
 };
 
 void wlr_touch_init(struct wlr_touch *touch,
-		struct wlr_touch_impl *impl);
+		const struct wlr_touch_impl *impl);
 void wlr_touch_destroy(struct wlr_touch *touch);
 
 #endif

include/wlr/types/wlr_touch.h

@@ -15,7 +15,7 @@
 struct wlr_touch_impl;
 
 struct wlr_touch {
-	struct wlr_touch_impl *impl;
+	const struct wlr_touch_impl *impl;
 
 	struct {
 		struct wl_signal down;

types/wlr_touch.c

@@ -5,7 +5,7 @@
 #include <wlr/types/wlr_touch.h>
 
 void wlr_touch_init(struct wlr_touch *touch,
-		struct wlr_touch_impl *impl) {
+		const struct wlr_touch_impl *impl) {
 	touch->impl = impl;
 	wl_signal_init(&touch->events.down);
 	wl_signal_init(&touch->events.up);