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>

Kbuild

diff --git a/Kbuild b/Kbuild
index e3737ad..18a8bfb 100644
--- a/Kbuild
+++ b/Kbuild
@@ -94,5 +94,5 @@
 missing-syscalls: scripts/checksyscalls.sh FORCE
 	$(call cmd,syscalls)
 
-# Delete all targets during make clean
-clean-files := $(addprefix $(objtree)/,$(filter-out $(bounds-file) $(offsets-file),$(targets)))
+# Keep these two files during make clean
+no-clean-files := $(bounds-file) $(offsets-file)