wayland: input: init axis_value120 to NULL

Fixes a missing initializer warning for axis_value120. We currently
enforce version = 5 for the wl_seat version, and it's available starting
with wl_pointer version 8, which means that we don't need to set a
function handler for now.

modules/video_output/wayland/input.c

@@ -206,6 +206,9 @@ static const struct wl_pointer_listener pointer_cbs =
     pointer_axis_source,
     pointer_axis_stop,
     pointer_axis_discrete,
+#ifdef WL_POINTER_AXIS_VALUE120_SINCE_VERSION
+    NULL, /* We don't support axis_value120 for now */
+#endif
 };
 
 static void pointer_create(struct seat_data *sd)