Explicitly include scene_tree.h where used

Pre-requisite before removing `scene_tree.h` from `node.h`
which has cascading consequences on most of the codebase.

modules/multiplayer/scene_cache_interface.cpp

@@ -35,6 +35,7 @@
 #include "core/io/marshalls.h"
 #include "core/object/callable_mp.h"
 #include "scene/main/node.h"
+#include "scene/main/scene_tree.h"
 #include "scene/main/window.h"
 
 SceneCacheInterface::NodeCache &SceneCacheInterface::_track(Node *p_node) {