Fix gnutls.c build on MinGW

Fixes: debbugs:19231

* gnutls.c (init_gnutls_functions, gnutls_certificate_details):
Use gnutls_sign_get_name directly because
gnutls_sign_algorithm_get_name is a macro on MinGW.
(init_gnutls_functions): Load missing gnutls_server_name_set.