fix default_timeout type of osmo_tdef_fsm_inst_state_chg default_timeout

The api doc indicates the possibility to pass -1, and calling
osmo_tdef_get() actually casts the arg to a signed long. To end the
confusion, change default_timeout from unsigned long to long.

Change-Id: I51b9172603984839448346c9836e43c8c802fcf8
diff --git a/src/tdef.c b/src/tdef.c
index 897a92f..9890f95 100644
--- a/src/tdef.c
+++ b/src/tdef.c
@@ -331,7 +331,7 @@
  */
 int _osmo_tdef_fsm_inst_state_chg(struct osmo_fsm_inst *fi, uint32_t state,
 				  const struct osmo_tdef_state_timeout *timeouts_array,
-				  const struct osmo_tdef *tdefs, unsigned long default_timeout,
+				  const struct osmo_tdef *tdefs, long default_timeout,
 				  const char *file, int line)
 {
 	const struct osmo_tdef_state_timeout *t = osmo_tdef_get_state_timeout(state, timeouts_array);