nios2: add FORCE for vmlinuz.gz

[ Upstream commit 869e4ae4cd2a23d625aaa14ae62dbebf768cb77d ]

Add FORCE to placate a warning from make:

arch/nios2/boot/Makefile:24: FORCE prerequisite is missing

Fixes: 2fc8483f ("nios2: Build infrastructure")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>