alembic: fix erroneous commit for add_prune_on_boot

Added include for postgresql ENUM type and
redefined values in the same way as in the
other migration scripts.

ASTERISK-27254 #close

Change-Id: Id667304cdf3891b1c2f7d35fab3e2a84026159fa

contrib/ast-db-manage/config/versions/f3d1c5d38b56_add_prune_on_boot.py

@@ -12,7 +12,10 @@ down_revision = '44ccced114ce'
 
 from alembic import op
 import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import ENUM
 
+YESNO_NAME = 'yesno_values'
+YESNO_VALUES = ['yes', 'no']
 
 def upgrade():
     ############################# Enums ##############################