pgsql: Add __attribute__((format_arg(1))) to the declaration of

Log Message:
-----------
Add __attribute__((format_arg(1))) to the declaration of err_gettext(),
to restore gcc's ability to crosscheck format arguments within elog.c.
Noted in a test compilation with -Wformat-nonliteral enabled.

Modified Files:
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pgsql/src/backend/utils/error:
elog.c (r1.215 -> r1.216)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/error/elog.c?r1=1.215&r2=1.216)