gsm48 lchan signal handling: use correct pointer

As Andreas Eversberg has pointed out, we need to use signal_data
rather than handler_data.
diff --git a/openbsc/src/gsm_04_08.c b/openbsc/src/gsm_04_08.c
index dc70c86..3b0b51d 100644
--- a/openbsc/src/gsm_04_08.c
+++ b/openbsc/src/gsm_04_08.c
@@ -385,7 +385,7 @@
 	 * Cancel any outstanding location updating request
 	 * operation taking place on the lchan.
 	 */
-	struct gsm_lchan *lchan = (struct gsm_lchan *)handler_data;
+	struct gsm_lchan *lchan = (struct gsm_lchan *)signal_data;
 	if (!lchan)
 		return 0;