Latest gnutls changes breaks --enable-gcc-warnings

[Dmitry Antipov <dmantipov@yandex.ru>]

As of e01ec2ed084776b370e0634120deec6b65424b8a, --enable-gcc-warnings produces:

../../emacs/src/gnutls.c:772:1: error: no previous prototype for ‘gnutls_hex_string’ [-Werror=missing-prototypes]
  gnutls_hex_string (char *buf, size_t buf_size, char *prefix) {
  ^
../../emacs/src/gnutls.c:790:1: error: no previous prototype for ‘gnutls_certificate_details’ [-Werror=missing-prototypes]
  gnutls_certificate_details (gnutls_x509_crt_t cert)
  ^
../../emacs/src/gnutls.c: In function ‘gnutls_certificate_details’:
../../emacs/src/gnutls.c:813:7: error: passing argument 3 of ‘gnutls_hex_string’ discards ‘const’ qualifier from pointer target type [-Werror]
        gnutls_hex_string (serial, serial_size, "")));
        ^
../../emacs/src/gnutls.c:772:1: note: expected ‘char *’ but argument is of type ‘const char *’
  gnutls_hex_string (char *buf, size_t buf_size, char *prefix) {
  ^
../../emacs/src/gnutls.c:927:9: error: passing argument 3 of ‘gnutls_hex_string’ discards ‘const’ qualifier from pointer target type [-Werror]
          gnutls_hex_string (buf, buf_size, "")));
          ^
../../emacs/src/gnutls.c:772:1: note: expected ‘char *’ but argument is of type ‘const char *’
  gnutls_hex_string (char *buf, size_t buf_size, char *prefix) {
  ^
../../emacs/src/gnutls.c:945:14: error: passing argument 3 of ‘gnutls_hex_string’ discards ‘const’ qualifier from pointer target type [-Werror]
               buf_size, "sha1:")));
               ^
../../emacs/src/gnutls.c:772:1: note: expected ‘char *’ but argument is of type ‘const char *’
  gnutls_hex_string (char *buf, size_t buf_size, char *prefix) {
  ^
../../emacs/src/gnutls.c:963:14: error: passing argument 3 of ‘gnutls_hex_string’ discards ‘const’ qualifier from pointer target type [-Werror]
               buf_size, "sha1:")));
               ^
../../emacs/src/gnutls.c:772:1: note: expected ‘char *’ but argument is of type ‘const char *’
  gnutls_hex_string (char *buf, size_t buf_size, char *prefix) {
  ^
../../emacs/src/gnutls.c: At top level:
../../emacs/src/gnutls.c:68:5: error: ‘extra_peer_verification_t’ defined but not used [-Werror=unused-variable]
    } extra_peer_verification_t;
      ^
cc1: all warnings being treated as errors

Dmitry