s390/decompressor: avoid packing *.o.chkbss files into startup.a

startup.a build rule packs a list of prerequisites into archive. That
didn't take into account extra prerequisites added by chkbss, so that
zero length *.o.chkbss files were also packed into the archive.

To avoid that filter only real object from prerequisites list.
Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/boot/Makefile

@@ -40,7 +40,7 @@ $(obj)/compressed/vmlinux: $(obj)/startup.a FORCE
 	$(Q)$(MAKE) $(build)=$(obj)/compressed $@
 
 quiet_cmd_ar = AR      $@
-      cmd_ar = rm -f $@; $(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(filter-out FORCE, $^)
+      cmd_ar = rm -f $@; $(AR) rcsTP$(KBUILD_ARFLAGS) $@ $(filter $(OBJECTS), $^)
 
 $(obj)/startup.a: $(OBJECTS) FORCE
 	$(call if_changed,ar)