efi/libstub: use 'targets' instead of extra-y in Makefile

These objects are built as prerequisites of %.stub.o files.
There is no need to use extra-y, which is planned for deprecation.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Author: masahir0y

drivers/firmware/efi/libstub/Makefile

@@ -102,7 +102,7 @@ lib-$(CONFIG_EFI_ZBOOT)		+= zboot.o $(zboot-obj-y)
 
 lib-$(CONFIG_UNACCEPTED_MEMORY) += unaccepted_memory.o bitmap.o find.o
 
-extra-y				:= $(lib-y)
+targets				:= $(lib-y)
 lib-y				:= $(patsubst %.o,%.stub.o,$(lib-y))
 
 # Even when -mbranch-protection=none is set, Clang will generate a