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.