Allow init to relabelto to misc_block_device.

avc:  denied  { relabelto } for  pid=1 comm="init" name="misc" dev="tmpfs" ino=3855 scontext=u:r:init:s0 tcontext=u:object_r:misc_block_device:s0 tclass=lnk_file

If misc partition is used during early mount, it will carry a label of
tmpfs (instead of block_device), which will fail restorecon with the
above denial.

Bug: 65378733
Test: Build and flash a target that uses misc in early mount. No longer
      observe the above denial.
Change-Id: I44cd43dbd2a8a4f9f423ebc8ac0dd046b167ef72