Fixed some clang warnings

src/slic3r/Utils/RetinaHelper.hpp

@@ -18,7 +18,9 @@ public:
     float get_scale_factor();
 
 private:
+#ifdef __WXGTK3__
     wxWindow* m_window;
+#endif // __WXGTK3__
     void* m_self;
 };
 

src/slic3r/Utils/RetinaHelperImpl.mm

@@ -11,8 +11,7 @@
 namespace Slic3r {
 namespace GUI {
 
-RetinaHelper::RetinaHelper(wxWindow* window) :
-    m_window(window)
+RetinaHelper::RetinaHelper(wxWindow*)
 {
     m_self = nullptr;
     m_self = [[RetinaHelperImpl alloc] initWithView:window->GetHandle() handler:window->GetEventHandler()];