device: Fixed CPP compatiblity for Z_DEVICE_HANDLES_DEFINE

By using the __weak attribute with the macro `Z_DEVICE_HANDLES_DEFINE`
C++ throws the error "weak declaration must be public". This change
adds the keyword extern if the macro is included in a C++ source
file.

Signed-off-by: Victor Chavez <chavez-bermudez@fh-aachen.de>