On a second thought, are you really sure we want to proceed with the header file?
After all the only line in it would be the extern declaration and the only one to
include it would be rp1-dev.c. Moreover, an header file would convey the false
premise that you can include it and use that symbol while in fact it should be
only used inside the driver.
OTOH, not creating that header file will continue to trigger the warning...

The header file does not need to be in global scope. It could be in
the driver source directory. As such, nothing outside of the driver
can use it.

Headers like this have multiple proposes. One is they make a symbol
visible to the linker. But having two different .c files include the
header enables type checking, which for long term maintenance is just
as important. So a one line header is fine.

	Andrew