Fix link error when disable_3d=yes

scene/main/viewport.cpp

@@ -894,6 +894,7 @@ void Viewport::_camera_remove(Camera* p_camera) {
 	}
 }
 
+#ifndef _3D_DISABLED
 void Viewport::_camera_make_next_current(Camera* p_exclude) {
 
 	for(Set<Camera*>::Element *E=cameras.front();E;E=E->next()) {
@@ -909,7 +910,7 @@ void Viewport::_camera_make_next_current(Camera* p_exclude) {
 
 	}
 }
-
+#endif
 
 void Viewport::set_transparent_background(bool p_enable) {