Include selinux_version file with builds.

The selinux_version file is used to perform policy
versioning checks by libselinux and SELinuxMMAC. When
loading policy a check is first performed to determine
if the policy out in /data/security/current should be
used to override the base policy shipped with the device.
The selinux_version file is used to make that choice. The
contents of the file simply contains the BUILD_ID that
the policy was built against. A simple string comparison
is then performed by libselinux and SELinuxMMAC.

Change-Id: I69d9d071743cfd46bb247c98f94a193396f8ebbd
Signed-off-by: rpcraig <rpcraig@tycho.ncsc.mil>