ppdev: add header include guard

Add a header include guard just in case.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Link: https://lore.kernel.org/r/20190728152739.9249-1-yamada.masahiro@socionext.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>