uncrypt: allow /dev/block directory access.

Uncrypt needs search in /dev/block to open block devices.
Allow it.

Addresses the following denial:

[11105.601711] type=1400 audit(1393550350.528:30): avc:  denied  { search } for  pid=14597 comm="uncrypt" name="block" dev="tmpfs" ino=7200 scontext=u:r:uncrypt:s0 tcontext=u:object_r:block_device:s0 tclass=dir

Change-Id: I4592784135a04ff5bff2715e1250661744f12aa1