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:
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pgsql/contrib/hstore:
hstore.sql.in (r1.5 -> r1.6)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/hstore/hstore.sql.in?r1=1.5&r2=1.6)