Ensure that SIZEOF_WCHAR_T is defined in wx/chartype.h

This should always the case now, but wasn't when not using configure
(e.g. in MSVC builds) before, so verify this explicitly to ensure that
we don't just silently define wxUSE_UNICODE_UTF16 wrongly, as it
happened before the fix in the previous commit.

include/wx/chartype.h

@@ -175,6 +175,10 @@
     #define wxUSE_UTF8_LOCALE_ONLY 0
 #endif
 
+#ifndef SIZEOF_WCHAR_T
+    #error "SIZEOF_WCHAR_T must be defined before including this file in wx/defs.h"
+#endif
+
 #if wxUSE_UNICODE_WCHAR && SIZEOF_WCHAR_T == 2
     #define wxUSE_UNICODE_UTF16 1
 #else