Avoid error in TLS connections due to incorrect format

* src/gnutls.c (Fgnutls_boot): Use the %x conversion specifier in
the call to 'error', instead of the unsupported %u.  Reported by
lo2net <fangtao0901@gmail.com>.  (Bug#20908)

src/gnutls.c

@@ -1512,7 +1512,7 @@ one trustfile (usually a CA bundle).  */)
           || !NILP (Fmember (QCgnutls_bootprop_trustfiles, verify_error)))
         {
 	  emacs_gnutls_deinit (proc);
-	  error ("Certificate validation failed %s, verification code %u",
+	  error ("Certificate validation failed %s, verification code %x",
 		 c_hostname, peer_verification);
         }
       else