CODEOWNERS: Add C++ library entry

This commit adds a CODEOWNERS entry for the C++ library (previously
called C++ subsystem) to appease the check_compliance.py.

Note that we have not completely phased out the CODEOWNERS file yet
and, until we do, the CODEOWNERS check in check_compliance.py should
stay.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>