package/systemd: enable selinux xdg module for refpolicy

Without the selinux xgd module enabled by systemd package, refpolicy
fail to build due to policy/modules/system/systemd.te [1]

policy/modules/system/systemd.te:288:ERROR 'attribute xdg_config_type is not declared' at token ';' on line 508447:

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/1710552468
https://gitlab.com/buildroot.org/buildroot/-/jobs/1710552470

[1] https://github.com/SELinuxProject/refpolicy/blob/RELEASE_2_20210908/policy/modules/system/systemd.te#L288



Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Adam Duskett <aduskett@gmail.com>
Cc: Antoine Tenart <atenart@kernel.org>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>