Alembic: Add enumerator value to sippeers -> directmedia - 'outgoing'

The 'outgoing' value was left off of the enumerator when first creating the
column. This patch adds it, and should gracefully upgrade keeping the existing
data in tact.

ASTERISK-23781 #close
Reported by: Stephen More
Review: https://reviewboard.asterisk.org/r/4013/



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@424372 f38db490-d61c-443f-a65b-d21fe96a405b