doc: suppress Clang warning in a snippet.

doc/snippets/MagnumSceneGraph.cpp

@@ -139,6 +139,10 @@ class CachingObject: public Object3D, SceneGraph::AbstractFeature3D {
 namespace {
 
 /* [transformation] */
+#ifdef __clang__
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-private-field"
+#endif
 class TransformingFeature: public SceneGraph::AbstractFeature3D {
     public:
         template<class T> explicit TransformingFeature(SceneGraph::Object<T>& object):
@@ -147,6 +151,9 @@ class TransformingFeature: public SceneGraph::AbstractFeature3D {
     private:
         SceneGraph::AbstractTranslationRotation3D& _transformation;
 };
+#ifdef __clang__
+#pragma GCC diagnostic pop
+#endif
 /* [transformation] */
 
 }