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.

scene/debugger/runtime_node_select.cpp

@@ -44,6 +44,7 @@
 #include "scene/debugger/scene_debugger_object.h"
 #include "scene/gui/popup_menu.h"
 #include "scene/main/canvas_layer.h"
+#include "scene/main/scene_tree.h"
 #include "scene/resources/mesh.h"
 #include "scene/theme/theme_db.h"
 #include "servers/rendering/rendering_server.h"