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.

openair2/LAYER2/MAC/eNB_scheduler_fairRR.c

@@ -52,7 +52,6 @@
 
 #ifdef PHY_TX_THREAD
   extern volatile int16_t phy_tx_txdataF_end;
-  extern int oai_exit;
 #endif
 extern uint16_t sfnsf_add_subframe(uint16_t frameP, uint16_t subframeP, int offset);
 extern void add_subframe(uint16_t *frameP, uint16_t *subframeP, int offset);