package/xscreensaver: needs xserver SELinux module

package/xscreensaver: needs xserver SELinux module

xserver is needed to fix the following build failure raised since commit
613827c5b0a2bf71320047954639dc9003524db0:

Compiling targeted policy.33
env LD_LIBRARY_PATH="/data/buildroot-autobuilder/instance-0/output-1/host/lib:/data/buildroot-autobuilder/instance-0/output-1/host/usr/lib" /data/buildroot-autobuilder/instance-0/output-1/host/usr/bin/checkpolicy -c 33 -U deny -S -O -E policy.conf -o policy.33
policy.conf:3071:ERROR 'attribute input_xevent_type is not declared' at token ';' on line 3071:
type xscreensaver_helper_t;
type xscreensaver_input_xevent_t, input_xevent_type, xevent_type;
checkpolicy:  error(s) encountered while parsing configuration
make[1]: *** [Rules.monolithic:79: policy.33] Error 1

Fixes:
 - http://autobuild.buildroot.org/results/2b5/2b546153ff8b9a11a5bb8475b918f220dd5b1787/build-end.log

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>