bridge core: Add owed T.38 terminate when channel leaves a bridge.

The channel is now going to get T.38 terminated when it leaves the
bridging system and the bridged peers are going to get T.38 terminated as
well.

ASTERISK-25582

Change-Id: I77a9205979910210e3068e1ddff400dbf35c4ca7

include/asterisk/bridge_channel.h

@@ -162,6 +162,8 @@ struct ast_bridge_channel {
 		struct timeval dtmf_tv;
 		/*! Digit currently sending into the bridge. (zero if not sending) */
 		char dtmf_digit;
+		/*! Non-zero if a T.38 session terminate is owed to the bridge. */
+		char t38_terminate;
 	} owed;
 	/*! DTMF hook sequence state */
 	struct {