lchan_rtp_fsm: fix out_state_mask

In the state LCHAN_RTP_ST_WAIT_MGW_ENDPOINT_CONFIGURED, the event
LCHAN_RTP_EV_ROLLBACK is allowed and also handled in the action
callback, which causes a change to LCHAN_RTP_ST_ROLLBACK. However,
LCHAN_RTP_ST_ROLLBACK is missing from the out_state_mask.

Change-Id: Ifca3892901c8389beee6e4f0fea03c33cfbdc265
diff --git a/src/osmo-bsc/lchan_rtp_fsm.c b/src/osmo-bsc/lchan_rtp_fsm.c
index 25aca14..4be9790 100644
--- a/src/osmo-bsc/lchan_rtp_fsm.c
+++ b/src/osmo-bsc/lchan_rtp_fsm.c
@@ -672,6 +672,7 @@
 			,
 		.out_state_mask = 0
 			| S(LCHAN_RTP_ST_READY)
+			| S(LCHAN_RTP_ST_ROLLBACK)
 			,
 	},
 	[LCHAN_RTP_ST_READY] = {