Organic groups patch to fix class override bug in subscription field

Organic groups 2.7 and actual dev version contain a bug that overrides
subscription field classes when it contains another #option attribute
insted of merge. drupal.org.make og module contains the local patch
until this bug is fixed in a stable upstream release.

Change-Id: Ic729e368c1eb07b56feef187bf3e1fe4c2cc31a2