GSMTAP: add function to create a 'sink' for gsmtap packets

This can be helpful where we send GSMTAP messages to the loopback
device (localhost, 127.0.0.1) from where the kernel would then
send ICMP reject packets as nobody is listening on that port.
diff --git a/include/osmocom/core/gsmtap_util.h b/include/osmocom/core/gsmtap_util.h
index 9644944..785f5e5 100644
--- a/include/osmocom/core/gsmtap_util.h
+++ b/include/osmocom/core/gsmtap_util.h
@@ -18,4 +18,8 @@
 
 int gsmtap_init(uint32_t dst_ip);
 
+/* Create a local 'gsmtap sink' avoiding the UDP packets being rejected
+ * with ICMP reject messages */
+int gsmtap_sink_init(uint32_t bind_ip);
+
 #endif /* _GSMTAP_UTIL_H */