Fix compiler warnings

I dared to remove the PROTOTYPE stuff from the MD5 implementation
as all the compilers we use support it.

Change-Id: I5a8c2acb3e61e176d85522f5310439b239cc154e
Reviewed-on: http://review.couchbase.org/30795
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>