sysmobts: Add the necessary data structure and init for the SAPI queue

The sysmobts is now having a SAPI queue with all pending SAPI operations
on the BTS. Add the llist_head to the lchan and make sure it is initialized
by the shared code.
diff --git a/openbsc/include/openbsc/gsm_data_shared.h b/openbsc/include/openbsc/gsm_data_shared.h
index f5ab473..a42b6bb 100644
--- a/openbsc/include/openbsc/gsm_data_shared.h
+++ b/openbsc/include/openbsc/gsm_data_shared.h
@@ -246,6 +246,7 @@
 #else
 	/* Number of different GsmL1_Sapi_t used in osmo_bts_sysmo is 23.
 	 * Currently we don't share these headers so this is a magic number. */
+	struct llist_head sapi_cmds;
 	uint8_t sapis_dl[23];
 	uint8_t sapis_ul[23];
 	struct lapdm_channel lapdm_ch;