Add qt6 as VCL backend fallback in gtk-based environments.
For details see: https://bugs.gentoo.org/950170

--- a/vcl/source/app/salplug.cxx
+++ b/vcl/source/app/salplug.cxx
@@ -196,6 +196,9 @@ std::vector<OUString> autodetect_plugin_list()
 #if ENABLE_GTK3
     aPlugins.push_back(u"gtk3"_ustr);
 #endif
+#if ENABLE_QT6
+    aPlugins.push_back(u"qt6"_ustr);
+#endif
 #if ENABLE_GEN
     aPlugins.push_back(u"gen"_ustr);
 #endif