Don't trigger warnings in standard headers when building wxQt

Pre-include QFont header in wxQt to avoid warnings for the other headers
included from it.

tests/allheaders.cpp

@@ -84,6 +84,10 @@
 
 #include "catch.hpp"
 
+#if defined(__WXQT__)
+    #include <QtGui/QFont>
+#endif
+
 #ifdef GCC_TURN_OFF
     #pragma GCC diagnostic pop
 #endif