http: fix compile-time warnings

Fix following warnings on RHEL6 and variants.

engines/http.c: In function 'fio_http_setup':
engines/http.c:611: warning: call to '_curl_easy_setopt_err_seek_cb' declared with attribute warning: curl_easy_setopt expects a curl_seek_callback argument for this option
engines/http.c: In function 'fio_http_queue':
engines/http.c:549: warning: call to '_curl_easy_setopt_err_curl_off_t' declared with attribute warning: curl_easy_setopt expects a curl_off_t argument for this option

For engines/http.c:611, pass &_http_seek instead of cast, since
lack of cast isn't fio's issue. See below comments for details.

"Re: [PATCH] http: fix compile-time warnings"
https://www.spinics.net/lists/fio/msg07246.html

Signed-off-by: Jens Axboe <axboe@kernel.dk>