meta-phosphor: hwmon: set as git recipe

Renamed the phosphor-hwmon.bb to phosphor-hwmon_git.bb.  This recipe is
old, however, a search through openbmc found all upstream layers
properly appended this recipe to allow for such a change.

(From meta-phosphor rev: 8f1d06b18710ccfb10b1a423991e9c617af6b6c6)

Change-Id: I5225e17ebe27899c7aead60627e036bea4822c85
Signed-off-by: Patrick Venture <venture@google.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>