Compilation fix for wxGTK with GTK+ 3.4 after recent changes

Fix the build after 1033fb048d.

See https://github.com/wxWidgets/wxWidgets/pull/320

src/unix/utilsx11.cpp

@@ -2615,7 +2615,7 @@ static bool wxGetKeyStateGTK(wxKeyCode key)
             break;
 
         default:
-            wxASSERT_MSG(wxS("Unsupported key, only modifiers can be used"));
+            wxFAIL_MSG(wxS("Unsupported key, only modifiers can be used"));
             return false;
     }
     return state & mask;