tools/include: Add pfn.h stub

Add a stubbed pfn header with definitions used in testing.

Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Signed-off-by: Mike Rapoport <rppt@kernel.org>
Link: https://lore.kernel.org/r/1bdc02125963f945bf90dd8cb37c404cc0688af2.1643796665.git.karolinadrobnik@gmail.com