meta-phosphor: set phosphor-host-postd as git recipe

Renamed the phosphor-host-postd.bb to phosphor-host-postd_git.bb.  This
recipe is new enough that such a change should have a minimal impact.

(From meta-phosphor rev: a9ac4e58eb8528ed0b2a7437c58867f01085f543)

Change-Id: I4de3fcb943209d2c77ee810791ff8abdc0647ca8
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>