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.

radio/ETHERNET/benetel/4g/benetel.c

@@ -243,7 +243,7 @@ next:
   //printf("BENETEL: %s (f.sf %d.%d)\n", __FUNCTION__, *frame, *subframe);
 
   RU_proc_t *proc = &ru->proc;
-  extern uint16_t sf_ahead;
+  extern int sf_ahead;
   int f = *frame;
   int sf = *subframe;
 

radio/ETHERNET/benetel/5g/benetel.c

@@ -257,7 +257,7 @@ printf("rxdata in benetel_fh_if4p5_south_in %p\n", &ru->common.rxdataF[antenna][
   //printf("BENETEL: %s (f.sf %d.%d)\n", __FUNCTION__, *frame, *slot);
 
   RU_proc_t *proc = &ru->proc;
-  extern uint16_t sl_ahead;
+  extern int sl_ahead;
   int f = *frame;
   int sl = *slot;