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

Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>

--- libreoffice-25.2.1.1.orig/vcl/source/app/salplug.cxx	2025-02-07 15:14:35.000000000 +0100
+++ libreoffice-25.2.1.1/vcl/source/app/salplug.cxx	2025-02-25 19:15:19.850379392 +0100
@@ -214,6 +214,9 @@ const char* const* autodetect_plugin_lis
 #if ENABLE_GTK3
         "gtk3",
 #endif
+#if ENABLE_QT6
+        "qt6",
+#endif
 #if ENABLE_GEN
         "gen",
 #endif