Test XRC headers compilation without implicit wxString conversion

Add wx/xrc/xmlres.h to the list of headers compiled with
wxNO_IMPLICIT_WXSTRING_ENCODING to test that they can be used even when
the implicit conversions from "char*" to wxString are disabled.

tests/allheaders.h

@@ -397,6 +397,7 @@
 #include <wx/wxprec.h>
 #include <wx/xlocale.h>
 #include <wx/xpmdecod.h>
+#include <wx/xrc/xmlres.h>
 #include <wx/xti2.h>
 #include <wx/xtictor.h>
 #include <wx/xti.h>