pgsql: Support functions for index opclasses should be immutable.

Log Message:
-----------
Support functions for index opclasses should be immutable.
Found by running opr_sanity on contrib modules.

Modified Files:
--------------
pgsql/contrib/pg_trgm:
pg_trgm.sql.in (r1.3 -> r1.4)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pg_trgm/pg_trgm.sql.in?r1=1.3&r2=1.4)