Fix harmless warning in wxString in wxUSE_UNICODE_UTF8 build

Add an explicit cast from ptrdiff_t to unsigned int to avoid the
warning which is harmless because the difference here is bound by the
cache size.

See #23297.

include/wx/string.h

@@ -553,7 +553,7 @@ private:
           c->Reset();
 
           // and remember the last used element
-          LastUsedCacheElement() = c - cacheBegin;
+          LastUsedCacheElement() = static_cast<unsigned int>(c - cacheBegin);
       }
 
       return c;