sparc: stop exporting openprom.h header

It's unknown why openprom.h was being exported; there doesn't seem to be any
reason for it currently, and it creates headaches with userspace being able
to potentially use the structures in there.  So, don't export it anymore.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>