Disable gtk in the build-config when cross-compiling.

Not needed at all and gtk3 is already disabled there.

Change-Id: Ic6f8be17645df22a414ae4b191a97b9bf1c16d1b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/90206
Tested-by: Jenkins
Reviewed-by: Jan Holesovsky <kendy@collabora.com>

configure.ac

@@ -4786,6 +4786,7 @@ if test "$cross_compiling" = "yes"; then
     ./configure \
         --disable-cups \
         --disable-gstreamer-1-0 \
+        --disable-gtk \
         --disable-gtk3 \
         --disable-pdfimport \
         --disable-postgresql-sdbc \