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/
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Merged revisions 424372 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 424373 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@424380 f38db490-d61c-443f-a65b-d21fe96a405b