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/main_ru.c

@@ -75,7 +75,7 @@ int sync_var=-1; //!< protected by mutex \ref sync_mutex.
 int config_sync_var=-1;
 
 int oai_exit = 0;
-uint16_t sf_ahead = 4;
+int sf_ahead = 4;
 RU_t ru_m;