package/pulseaudio: needs xdg SELinux module

xdg is needed (and can't be make optional as it is unconditionally used
in pulseaudio.fc) to fix the following build failure raised since commit
bf44a11c:

 Compiling targeted policy.33
 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 33 -U deny -S -O -E policy.conf -o policy.33
 policy/modules/apps/pulseaudio.te:44:ERROR 'attribute xdg_config_type is not declared' at token ';' on line 317285:
 #line 44
	typeattribute pulseaudio_xdg_config_t xdg_config_type;
 checkpolicy:  error(s) encountered while parsing configuration
 make[1]: *** [Rules.monolithic:79: policy.33] Error 1

Fixes:
 - http://autobuild.buildroot.org/results/818219c0f722080d9f6ef778fdc50e34dd4187ab



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