package/xscreensaver: add xdg SELinux module

xdg SELinux module is needed to avoid the following build failure raised
since commit 62b4fa2d7b543cfbacbc8fa112a88fa575b23eb3:

 Compiling targeted policy.31
 env LD_LIBRARY_PATH="/home/buildroot/autobuild/instance-2/output-1/host/lib:/home/buildroot/autobuild/instance-2/output-1/host/usr/lib" /home/buildroot/autobuild/instance-2/output-1/host/usr/bin/checkpolicy -c 31 -U deny -S -O -E policy.conf -o policy.31
 policy/modules/services/xserver.te:116:ERROR 'attribute xdg_cache_type is not declared' at token ';' on line 656367:
 #line 116
	typeattribute user_fonts_cache_t xdg_cache_type;
 checkpolicy:  error(s) encountered while parsing configuration

Fixes:
 - http://autobuild.buildroot.org/results/610484c288fa1923d9ee28a3cb780dcb67579eee

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/xscreensaver/xscreensaver.mk

@@ -11,7 +11,7 @@ XSCREENSAVER_SITE = https://www.jwz.org/xscreensaver
 XSCREENSAVER_LICENSE = MIT-like, GPL-2.0+
 XSCREENSAVER_LICENSE_FILES = hacks/screenhack.h hacks/glx/chessmodels.h
 XSCREENSAVER_CPE_ID_VENDOR = xscreensaver_project
-XSCREENSAVER_SELINUX_MODULES = xscreensaver xserver
+XSCREENSAVER_SELINUX_MODULES = xdg xscreensaver xserver
 
 XSCREENSAVER_DEPENDENCIES = \
 	gdk-pixbuf \