Fix compile errors with OpenSSL 1.1.0.
HMAC_CTX is made opaque and HMAC_CTX_cleanup is removed in OpenSSL
1.1.0. But since we just want to calculate one HMAC, we can use HMAC()
here, which exists since OpenSSL 0.9.6 at least.
Signed-off-by: Kazuki Yamaguchi <k@rhe.jp>
Signed-off-by: Junio C Hamano <gitster@pobox.com>