sepolicy : add rule to allow dpmd self kill

add self kill rule to kill dpmd child process
which executes iptable commands.

denial:
dpmd    : type=1400 audit(0.0:56633): avc: denied { kill }
for capability=5 scontext=u:r:dpmd:s0 tcontext=u:r:dpmd:s0
tclass=capability permissive=1

CRs-Fixed: 2490550
Change-Id: I4cc1f23a8c3ba37e33fd02c729c2f4d2a7eea4d4