Include crc32.h only once

Change-Id: I4d60fe748431ad3602456b3e9854b53a1c438fe5
Reviewed-on: http://review.couchbase.org/27675
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>