NULL isn't standard thing, so require stdlib.h

Change-Id: Ib92fef78fd2c4164e9394ab7b640501308f064cb
Reviewed-on: http://review.couchbase.org/25286
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>