domain: allow dir search in selinuxfs

Domain is already allowed to stat selinuxfs, it also needs
dir search.

Addresses:
avc: denied { search } for name="/" dev="selinuxfs" ino=1 scontext=u:r:watchdogd:s0 tcontext=u:object_r:selinuxfs:s0 tclass=dir

Change-Id: I3e5bb96e905db480a2727038f80315d9544e9c07