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/gltf/tests/test_gltf_extras.h

@@ -38,6 +38,7 @@
 #include "editor/import/3d/resource_importer_scene.h"
 #include "scene/3d/mesh_instance_3d.h"
 #include "scene/3d/skeleton_3d.h"
+#include "scene/main/scene_tree.h"
 #include "scene/main/window.h"
 #include "scene/resources/3d/primitive_meshes.h"
 #include "scene/resources/material.h"

modules/gltf/tests/test_gltf_images.h

@@ -36,6 +36,7 @@
 
 #include "editor/file_system/editor_file_system.h"
 #include "editor/file_system/editor_paths.h"
+#include "scene/main/scene_tree.h"
 #include "scene/resources/image_texture.h"
 
 namespace TestGltf {