Change lcb_http_callback to lcb_http_res_callback

Otherwise it conflicts with a symbol defined in the php ext with the
same name.

Change-Id: Ic0d0d1bfc75fecd03831e3c6401d52eaecaf944e
Reviewed-on: http://review.couchbase.org/40033
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>