s390/decompressor: add missing FORCE to build targets

s390/decompressor: add missing FORCE to build targets

[ Upstream commit ef5febae1543f35a45f01614123e829d77326d0f ]

According to Documentation/kbuild/makefiles.txt all build targets
using if_changed should use FORCE as well. Add missing FORCE to make
sure vmlinux decompressor targets are rebuild properly when not just
immediate prerequisites have changed but also when build command differs.

Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>