example compositor: handle keyboard after we're done with init

the libinput backend does wl_display roundtrips, during which there is a
small window where keystrokes can be handled before the rest of the example
compositor is ready.
Setting the callback later ensures we're not called at this point

examples/compositor.c

@@ -143,7 +143,6 @@ int main() {
 	struct compositor_state compositor = { 0,
 		.data = &state,
 		.output_frame_cb = handle_output_frame,
-		.keyboard_key_cb = handle_keyboard_key
 	};
 	compositor_init(&compositor);
 
@@ -178,6 +177,8 @@ int main() {
 		break;
 	}
 
+	compositor.keyboard_key_cb = handle_keyboard_key;
+
 	wl_display_run(compositor.display);
 
 	close(state.keymap_fd);