sparc: Add missing "FORCE" target when using if_changed

Fix observed warning:

    /builds/linux/arch/sparc/boot/Makefile:35: FORCE prerequisite is missing

Fixes: e1f86d7b4b ("kbuild: warn if FORCE is missing for if_changed(_dep,_rule) and filechk")
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

arch/sparc/boot/Makefile

@@ -22,7 +22,7 @@ ifeq ($(CONFIG_SPARC64),y)
 
 # Actual linking
 
-$(obj)/zImage: $(obj)/image
+$(obj)/zImage: $(obj)/image FORCE
 	$(call if_changed,gzip)
 	@echo '  kernel: $@ is ready'
 
@@ -31,7 +31,7 @@ $(obj)/vmlinux.aout: vmlinux FORCE
 	@echo '  kernel: $@ is ready'
 else
 
-$(obj)/zImage: $(obj)/image
+$(obj)/zImage: $(obj)/image FORCE
 	$(call if_changed,strip)
 	@echo '  kernel: $@ is ready'
 
@@ -44,7 +44,7 @@ OBJCOPYFLAGS_image.bin := -S -O binary -R .note -R .comment
 $(obj)/image.bin: $(obj)/image FORCE
 	$(call if_changed,objcopy)
 
-$(obj)/image.gz: $(obj)/image.bin
+$(obj)/image.gz: $(obj)/image.bin FORCE
 	$(call if_changed,gzip)
 
 UIMAGE_LOADADDR = $(CONFIG_UBOOT_LOAD_ADDR)
@@ -56,7 +56,7 @@ quiet_cmd_uimage.o = UIMAGE.O $@
                      -r -b binary $@ -o $@.o
 
 targets += uImage
-$(obj)/uImage: $(obj)/image.gz
+$(obj)/uImage: $(obj)/image.gz FORCE
 	$(call if_changed,uimage)
 	$(call if_changed,uimage.o)
 	@echo '  Image $@ is ready'