efi/libstub: zboot: Add compressed image to make targets

Avoid needlessly rebuilding the compressed image by adding the file
'vmlinuz' to the 'targets' Kbuild make variable.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/libstub/Makefile.zboot

@@ -44,4 +44,4 @@ OBJCOPYFLAGS_vmlinuz.efi := -O binary
 $(obj)/vmlinuz.efi: $(obj)/vmlinuz.efi.elf FORCE
 	$(call if_changed,objcopy)
 
-targets += zboot-header.o vmlinuz.o vmlinuz.efi.elf vmlinuz.efi
+targets += zboot-header.o vmlinuz vmlinuz.o vmlinuz.efi.elf vmlinuz.efi