sip: use ipv4 exclusively for now

IPv6 should work, but sofia's outbound engine keep printing
errors involving the outbound engine. Working theory:
Failing ICMPv6 (pings) can make sofia think we don't have connectivity.
Note that we also don't get any answers from the SIP servers we tried so far.

plugins/sip/calls-sip-util.c

@@ -24,6 +24,14 @@
 
 #include "calls-sip-util.h"
 
+gboolean
+check_ipv6 (const gchar *host)
+{
+  /* TODO DNS SRV records to determine whether or not to use IPv6 */
+  return FALSE;
+}
+
+
 gboolean
 check_sips (const gchar *addr)
 {