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-uesoftmodem.c

@@ -73,8 +73,7 @@ pthread_cond_t nfapi_sync_cond;
 pthread_mutex_t nfapi_sync_mutex;
 int nfapi_sync_var=-1; //!< protected by mutex \ref nfapi_sync_mutex
 
-
-uint16_t sf_ahead=4;
+int sf_ahead = 4;
 int tddflag;
 char *emul_iface;