pgsql-server: Fix oversight in recent ALTER TABLE improvements.

Log Message:
-----------
Fix oversight in recent ALTER TABLE improvements.  We now support
ALTER TABLE tab ADD COLUMN col SERIAL, but we forgot to install the
dependency between the column and the sequence, so the sequence
would not go away if you dropped the table later.

Modified Files:
--------------
    pgsql-server/src/backend/commands:
        tablecmds.c (r1.113 -> r1.114)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/tablecmds.c.diff?r1=1.113&r2=1.114)