Fixed ZooKeeper 3.4.5 OSX Yosemite build.

Fixes build issue caused by htonll introduced by OSX 10.10 (Yosemite).

This poses as a hotfix to prevent us from having to upgrade the bundled
ZooKeeper altogether to prevent longer regression testing. Once
ZooKeeper does get updated into a recent version (3.4.x >= 3.4.7,
3.5.x >= 3.5.1 or >= 3.6.0), this patch should get removed.

Review: https://reviews.apache.org/r/26571