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/TODO-RELEASE b/TODO-RELEASE
index 7ba9347..c5d4ee8 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -10,3 +10,4 @@
 libosmovty	_LAST_OSMOVTY_NODE 	Raise _LAST_OSMOVTY_NODE by introducing some RESERVED*_NODE
 libosmogsm      gsm0808_old_bss_to_new_bss_info ABI break (struct changes size), gsm0808_old_bss_to_new_bss_info_att_tlvdef symbol added
 libosmosim	osim_card_hdl		ABI + API breakage due to new struct members
+libosmocore	osmo_tdef_fsm_inst_state_chg	change default_timeout arg from unsigned long to long type (API breakage, not ABI)