Work around gcc warnings from curl headers

Work around gcc warnings from curl headers

After master.k.org upgrade, I started seeing these warning messages:

    transport.c: In function 'get_refs_via_curl':
    transport.c:458: error: call to '_curl_easy_setopt_err_write_callback' declared with attribute warning: curl_easy_setopt expects a curl_write_callback argument for this option

It appears that the curl header wants to enforce the function signature
for callback function given to curl_easy_setopt() to be compatible with
that of (*curl_write_callback) or fwrite.  This patch seems to work the
issue around.

Signed-off-by: Junio C Hamano <gitster@pobox.com>