lib/util/charset Use top level iconv.c in source3

lib/util/charset Use top level iconv.c in source3

The two files were very similar already, the only change required was
to adopt the s3 module registration fucntion name.

(NTSTATUS wasn't used as the charset code does not otherwise use that
type).

Andrew Bartlett

Signed-off-by: Andrew Tridgell <tridge@samba.org>