curl: Cast fd to int for DPRINTF

curl: Cast fd to int for DPRINTF

Currently "make docker-test-mingw@fedora" has a warning like:

    /tmp/qemu-test/src/block/curl.c: In function 'curl_sock_cb':
    /tmp/qemu-test/src/block/curl.c:172:6: warning: format '%d' expects
    argument of type 'int', but argument 4 has type 'curl_socket_t {aka long
    long unsigned int}'
         DPRINTF("CURL (AIO): Sock action %d on fd %d\n", action, fd);
          ^
    cc1: all warnings being treated as errors

Cast to int to suppress it.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1470027888-24381-1-git-send-email-famz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>