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

Makefile

@@ -1170,7 +1170,7 @@ MRPROPER_FILES += .config .config.old .version .old_version             \
 #
 clean: rm-dirs  := $(CLEAN_DIRS)
 clean: rm-files := $(CLEAN_FILES)
-clean-dirs      := $(addprefix _clean_, . $(vmlinux-alldirs) Documentation)
+clean-dirs      := $(addprefix _clean_, . $(vmlinux-alldirs) Documentation samples)
 
 PHONY += $(clean-dirs) clean archclean
 $(clean-dirs):