Fix wxX11 build with wxUSE_STL=1

Add missing utf8_str() call as implicit conversion is not available in
wxUSE_STL=1 case (notice that this also makes wxUSE_STL=0 work correctly when
not using UTF-8 locale).

Closes #17412.

src/x11/dcclient.cpp

@@ -1679,7 +1679,7 @@ void wxWindowDCImpl::DoDrawRotatedText(const wxString& text,
     // and wxWidgets is counter clock, so plus minus.
     cairo_rotate(cr, -angle * (M_PI / 180));
 
-    cairo_show_text(cr, text);
+    cairo_show_text(cr, text.utf8_str());
     cairo_restore(cr);
     cairo_destroy(cr);
 }