Workaround for gtk3 plugin issues

--- libreoffice-6.4.3.2/desktop/scripts/soffice.sh.1	2020-04-27 10:58:43.712730112 +0000
+++ libreoffice-6.4.3.2/desktop/scripts/soffice.sh	2020-04-27 11:03:59.109348940 +0000
@@ -27,6 +27,10 @@
 # export STAR_PROFILE_LOCKING_DISABLED
 #
 
+# GTK3 plugin is not usable on OI and needs more work, so use qt6 plugin for now
+SAL_USE_VCLPLUGIN=${SAL_USE_VCLPLUGIN:-"qt6"}
+export SAL_USE_VCLPLUGIN
+
 # file locking now enabled by default
 SAL_ENABLE_FILE_LOCKING=1
 export SAL_ENABLE_FILE_LOCKING