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/include/osmocom/core/tdef.h b/include/osmocom/core/tdef.h
index 627ba3f..d9d2675 100644
--- a/include/osmocom/core/tdef.h
+++ b/include/osmocom/core/tdef.h
@@ -162,7 +162,7 @@
 				      __FILE__, __LINE__)
 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, signed long default_timeout,
 				  const char *file, int line);
 
 /*! Manage timer definitions in named groups.