Fixes link error when a CAN driver is written in c++: adds extern "C" guards to can.h.
Fixes link error when a CAN driver is written in c++: adds extern "C" guards to can.h.

bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn,bakerstu/openmrn