[sccp] Implement sending the Inactivity Test on a connection..

Currently this will send a dummy inactivity test message,
there is currently no parsing or API to receive the messages.

The sequencing and credit entries are empty as sequencing
is currently not used at all.

The test is currently limited to send the message and see
if the application is crashing or not.
diff --git a/openbsc/include/sccp/sccp.h b/openbsc/include/sccp/sccp.h
index 8ee4b68..3ad568c 100644
--- a/openbsc/include/sccp/sccp.h
+++ b/openbsc/include/sccp/sccp.h
@@ -101,6 +101,7 @@
  * Send data on an existing connection
  */
 int sccp_connection_write(struct sccp_connection *connection, struct msgb *data);
+int sccp_connection_send_it(struct sccp_connection *connection);
 int sccp_connection_close(struct sccp_connection *connection, int cause);
 int sccp_connection_free(struct sccp_connection *connection);