rtio: Make rtio_mpsc.h C++ friendly

C++ requires casting void * as the implicit cast isn't enough and
the C++ sample app fails to build without duplicating type information
here. Do that, and wrap it in extern C allowing the C++ to include
rtio_mpsc.h directly or indirectly.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>