freetdm: add release event to ftdm core

libs/freetdm/src/include/freetdm.h

@@ -287,6 +287,7 @@ typedef enum {
 typedef enum {
 	FTDM_SIGEVENT_START, /*!< Incoming call (ie: incoming SETUP msg or Ring) */
 	FTDM_SIGEVENT_STOP, /*!< Hangup */
+	FTDM_SIGEVENT_RELEASED, /*!< Channel is completely released and available */
 	FTDM_SIGEVENT_UP, /*!< Outgoing call has been answered */
 	FTDM_SIGEVENT_FLASH, /*< Flash event  (typically on-hook/off-hook for analog devices) */
 	FTDM_SIGEVENT_PROGRESS, /*!< Outgoing call is making progress */