libdns: Provide replacement for EPROTO.

* dirmngr/dns.c (EPROTO) ![EPROTO]: Define to EPROTONOSUPPORT.
--

This is the same replacement we use in Libassuan
(commit 8ab3b9273524bd344bdb90dd5d3bc8e5f53ead6e) to make it work on
OpenBSD and may other BSD based OSes.

Signed-off-by: Werner Koch <wk@gnupg.org>

dirmngr/dns.c

@@ -288,6 +288,10 @@ int dns_v_api(void) {
  *
  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 
+#ifndef EPROTO
+# define EPROTO EPROTONOSUPPORT
+#endif
+
 #if _WIN32
 
 #define DNS_EINTR	WSAEINTR