pgsql: Tweak clean_encoding_name() API to avoid need to cast away const.

Log Message:
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Tweak clean_encoding_name() API to avoid need to cast away const.
Kris Jurka

Modified Files:
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pgsql/src/backend/utils/mb:
encnames.c (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/mb/encnames.c.diff?r1=1.33&r2=1.34)