Fix packaging of unit test files

Fix packaging of unit test files

Previously, the unit test sysfs mock files and headers were not
added to the distribution packag. Not packaging the header leads
to compilation of the unit tests failing. Not packaging the stub
files caueses the unit tests themselves to fail.

This patch explicitly adds the header and sysfs mock files to the
distribution target, allowing the unit tests to be used outside
the git tree.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>