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.h

@@ -80,6 +80,7 @@ typedef enum
 
 
 gboolean check_sips (const gchar *addr);
+gboolean check_ipv6 (const gchar *host);
 const gchar *get_protocol_prefix (const gchar *protocol);
 gboolean protocol_is_valid (const gchar *protocol);
 guint get_port_for_rtp (void);