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/nr-ru.c

@@ -69,7 +69,7 @@ static int DEFRUTPCORES[] = {-1,-1,-1,-1};
 #include <nfapi/oai_integration/vendor_ext.h>
 #include "executables/nr-softmodem-common.h"
 
-uint16_t sl_ahead;
+int sl_ahead;
 static void NRRCconfig_RU(configmodule_interface_t *cfg);
 
 /*************************************************************/