selinux: explicitly declare the role "base_r"

This fixes the compilation of policy generated by mdp with the recent
version of checkpolicy.

Signed-off-by: Laurent Bigonville <bigon@bigon.be>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <pmoore@redhat.com>