sysmobts: Add variable for N(S) + 1 needed for the sysmobts

We could use the upper three bits of the ciph_state to store the
sequence number but make the code understandable first.

openbsc/include/openbsc/gsm_data_shared.h

@@ -295,6 +295,7 @@ struct gsm_lchan {
 	} tch;
 	/* BTS-side ciphering state (rx only, bi-directional, ...) */
 	uint8_t ciph_state;
+	uint8_t ciph_ns;
 	uint8_t loopback;
 	struct {
 		uint8_t active;