Fix compiling SkeletonIK with disable_3d

Fixes #20825.

scene/animation/skeleton_ik.cpp

@@ -34,6 +34,8 @@
 
 #include "skeleton_ik.h"
 
+#ifndef _3D_DISABLED
+
 FabrikInverseKinematic::ChainItem *FabrikInverseKinematic::ChainItem::find_child(const BoneId p_bone_id) {
 	for (int i = childs.size() - 1; 0 <= i; --i) {
 		if (p_bone_id == childs[i].bone) {
@@ -549,3 +551,5 @@ void SkeletonIK::_solve_chain() {
 		return;
 	FabrikInverseKinematic::solve(task, interpolation, use_magnet, magnet_position);
 }
+
+#endif // _3D_DISABLED

scene/animation/skeleton_ik.h

@@ -31,6 +31,8 @@
 #ifndef SKELETON_IK_H
 #define SKELETON_IK_H
 
+#ifndef _3D_DISABLED
+
 /**
  * @author AndreaCatania
  */
@@ -209,4 +211,6 @@ private:
 	void _solve_chain();
 };
 
+#endif // _3D_DISABLED
+
 #endif // SKELETON_IK_H

scene/register_scene_types.cpp

@@ -51,7 +51,6 @@
 #include "scene/2d/parallax_background.h"
 #include "scene/2d/parallax_layer.h"
 #include "scene/2d/particles_2d.h"
-
 #include "scene/2d/path_2d.h"
 #include "scene/2d/physics_body_2d.h"
 #include "scene/2d/polygon_2d.h"
@@ -366,14 +365,12 @@ void register_scene_types() {
 	ClassDB::register_class<Spatial>();
 	ClassDB::register_virtual_class<SpatialGizmo>();
 	ClassDB::register_class<Skeleton>();
-	ClassDB::register_class<SkeletonIK>();
 	ClassDB::register_class<AnimationPlayer>();
 	ClassDB::register_class<Tween>();
 
 	OS::get_singleton()->yield(); //may take time to init
 
 #ifndef _3D_DISABLED
-	ClassDB::register_class<BoneAttachment>();
 	ClassDB::register_virtual_class<VisualInstance>();
 	ClassDB::register_virtual_class<GeometryInstance>();
 	ClassDB::register_class<Camera>();
@@ -438,6 +435,9 @@ void register_scene_types() {
 	ClassDB::register_class<PhysicalBone>();
 	ClassDB::register_class<SoftBody>();
 
+	ClassDB::register_class<SkeletonIK>();
+	ClassDB::register_class<BoneAttachment>();
+
 	ClassDB::register_class<VehicleBody>();
 	ClassDB::register_class<VehicleWheel>();
 	ClassDB::register_class<Area>();