kbuild: clean up samples directory

Playing around with make randconfig I ended up with CONFIG_SAMPLES
set. After cleaning up the build I noticed that git status was
showing samples/hidraw/hid-example as an untracked file.

Adding samples to clean-dirs seems to solve the problem, but
I don't know if that is the correct way to solve the issue.

Signed-off-by: Gerard Snitselaar <dev@snitselaar.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>