kbuild: Really don't clean bounds.h and asm-offsets.h

Commit 7d3cc8b tried to keep bounds.h and asm-offsets.h during make
clean by filtering these out of $(clean-files), but they are listed in
$(targets) and $(always) and thus removed automatically. Introduce a new
$(no-clean-files) variable to really skip such files in Makefile.clean.

Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/scripts/Makefile.clean b/scripts/Makefile.clean
index 6f89fbb..686cb0d 100644
--- a/scripts/Makefile.clean
+++ b/scripts/Makefile.clean
@@ -45,6 +45,8 @@
 		   $(host-progs)                         \
 		   $(hostprogs-y) $(hostprogs-m) $(hostprogs-)
 
+__clean-files   := $(filter-out $(no-clean-files), $(__clean-files))
+
 # as clean-files is given relative to the current directory, this adds
 # a $(obj) prefix, except for absolute paths