package/x11r7/xserver_xorg-server: add xdg SELinux module

xdg SELinux module is needed to avoid the following build failure raised
since commit 5220d3c5c62df35e994b4b7c6caecc8d16e72b78:

 Compiling targeted policy.30
 env LD_LIBRARY_PATH="/tmp/instance-0/output-1/host/lib:/tmp/instance-0/output-1/host/usr/lib" /tmp/instance-0/output-1/host/usr/bin/checkpolicy -c 30 -U deny -S -O -E policy.conf -o policy.30
 policy/modules/services/xserver.te:116:ERROR 'attribute xdg_cache_type is not declared' at token ';' on line 632811:
	typeattribute user_fonts_cache_t xdg_cache_type;
 checkpolicy:  error(s) encountered while parsing configuration

Fixes:
 - http://autobuild.buildroot.org/results/a1222d4ba0abf104dfc4a3dfc3c787e843fb6124

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/x11r7/xserver_xorg-server/xserver_xorg-server.mk

@@ -9,7 +9,7 @@ XSERVER_XORG_SERVER_SOURCE = xorg-server-$(XSERVER_XORG_SERVER_VERSION).tar.xz
 XSERVER_XORG_SERVER_SITE = https://xorg.freedesktop.org/archive/individual/xserver
 XSERVER_XORG_SERVER_LICENSE = MIT
 XSERVER_XORG_SERVER_LICENSE_FILES = COPYING
-XSERVER_XORG_SERVER_SELINUX_MODULES = xserver
+XSERVER_XORG_SERVER_SELINUX_MODULES = xdg xserver
 XSERVER_XORG_SERVER_INSTALL_STAGING = YES
 # xfont_font-util is needed only for autoreconf
 XSERVER_XORG_SERVER_AUTORECONF = YES