Include <arpa/inet.h> when checking for htonll

Otherwise we clobber htonll in OS X Yosemite

Change-Id: Ica6b24e6752e7f4d68974ea613d460d8b94bacea
Reviewed-on: http://review.couchbase.org/42283
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>