shared: Introduce/Add procedure pending to the MO

GSM 12.21 specifies that "No elementary procedure shall be
initiated to an object instance which has not yet replied to
a previously initiated elementary procedure with a response,
an ACK or a NACK within a layer 3 time-out. The layer 3
timeout for ACK, NACK and responses shall have a default value
of 10 seconds."

We are using this flag in the BTS to enforce/safe-guard
this situation.

openbsc/include/openbsc/gsm_data_shared.h

@@ -95,6 +95,7 @@ struct gsm_nm_state {
 
 struct gsm_abis_mo {
 	uint8_t obj_class;
+	uint8_t procedure_pending;
 	struct abis_om_obj_inst obj_inst;
 	const char *name;
 	struct gsm_nm_state nm_state;