s390/decompressor: add missing FORCE to build targets

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>