Further cleanup: define oai_exit for 4G

The parent commit cleans up and groups oai_exit in one place,
nr-softmodem-common.h. Therefore, this 5G file is included in the 4G
defs_common.h (at least it's mostly 4G), which is not nice. Instead,
define oai_exit also in 4G softmodem-common.h, and include this in
defs_common.h. Remove the cyclic inclusion of defs_common.h in
softmodem-common.h, and resolve all subsequent errors around unknown
types and extern definition mismatches.

executables/lte-ue.c

@@ -79,7 +79,7 @@ void *UE_thread(void *arg);
 int init_timer_thread(void);
 
 int tx_req_num_elems;
-extern uint16_t sf_ahead;
+extern int sf_ahead;
 //extern int tx_req_UE_MAC1();
 
 void ue_stub_rx_handler(unsigned int, char *);