Revert "remove typedef `Decoration`"

This reverts commit d1ff1e6f75.

dwl.c

@@ -133,6 +133,11 @@ typedef struct {
 	uint32_t resize; /* configure serial of a pending resize */
 } Client;
 
+typedef struct {
+	struct wl_listener request_mode;
+	struct wl_listener destroy;
+} Decoration;
+
 typedef struct {
 	uint32_t mod;
 	xkb_keysym_t keysym;