DebugTools: no need to set hidden visibility for a nested class.

It makes no difference in this case, the State struct is not exported
even without the attribute. OTOH, it would need to have an EXPORT
attribute if it was desired to be exported, for example like done with
nested classes in GL::Framebuffer.

src/Magnum/DebugTools/CompareImage.h

@@ -92,7 +92,7 @@ class MAGNUM_DEBUGTOOLS_EXPORT ImageComparatorBase {
         void saveDiagnostic(TestSuite::ComparisonStatusFlags flags, Utility::Debug& out, Containers::StringView path);
 
     private:
-        class MAGNUM_DEBUGTOOLS_LOCAL State;
+        class State;
         Containers::Pointer<State> _state;
 };