Don't require vcl.unx component on headless build

Change-Id: I33c3d90b67dd4ef3fc52ae2555b41ca2deb191de
Reviewed-on: https://gerrit.libreoffice.org/12267Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>