nios2: add FORCE for vmlinuz.gz

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>

arch/nios2/boot/Makefile

@@ -20,7 +20,7 @@ $(obj)/vmlinux.bin: vmlinux FORCE
 $(obj)/vmlinux.gz: $(obj)/vmlinux.bin FORCE
 	$(call if_changed,gzip)
 
-$(obj)/vmImage: $(obj)/vmlinux.gz
+$(obj)/vmImage: $(obj)/vmlinux.gz FORCE
 	$(call if_changed,uimage)
 	@$(kecho) 'Kernel: $@ is ready'