Add osmo_fsm_unregister() to header

Previously function was defined but not exposed so there were a way to
register FSM but no way to unregister it.

Change-Id: I2e749d896009784b77d6d5952fcc38e1c131db2b
diff --git a/tests/fsm/fsm_test.c b/tests/fsm/fsm_test.c
index 84c2ae4..29b31ef 100644
--- a/tests/fsm/fsm_test.c
+++ b/tests/fsm/fsm_test.c
@@ -151,5 +151,6 @@
 		osmo_select_main(0);
 	}
 
+	osmo_fsm_unregister(&fsm);
 	exit(0);
 }