Silence warning building with macOS SDK < 10.14

src/osx/carbon/utilscocoa.mm

@@ -555,6 +555,8 @@ wxOSXEffectiveAppearanceSetter::wxOSXEffectiveAppearanceSetter()
         formerAppearance = NSAppearance.currentAppearance;
         NSAppearance.currentAppearance = NSApp.effectiveAppearance;
     }
+#else
+    wxUnusedVar(formerAppearance);
 #endif
 }