libpayload: Include libpayload-config.h in lib target

- Added `$(obj)/libpayload-config.h` as a dependency for the `lib`
  target.
- This ensures the config header is up-to-date before building the
  library.

TEST=Able to build google/rex.

Change-Id: If26336f6261aadf611fa5338c4300873156cc3da
Signed-off-by: Subrata Banik <subratabanik@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/82687
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Dinesh Gehlot <digehlot@google.com>
Reviewed-by: Kapil Porwal <kapilporwal@google.com>

payloads/libpayload/Makefile.mk

@@ -115,7 +115,7 @@ $(obj)/%.a: $$(%-objs)
 	printf "    AR         $(subst $(CURDIR)/,,$(@))\n"
 	printf "create $@\n$(foreach objc,$(filter-out %.a,$^),addmod $(objc)\n)$(foreach lib,$(filter %.a,$^),addlib $(lib)\n)save\nend\n" | $(AR) -M
 
-$(obj)/libpayload.ldscript: arch/$(ARCHDIR-y)/libpayload.ldscript
+$(obj)/libpayload.ldscript: arch/$(ARCHDIR-y)/libpayload.ldscript $(obj)/libpayload-config.h
 	@printf "  LDSCRIPT  $@\n"
 	$(CC) $(CFLAGS) $(EXTRA_CFLAGS) -E -P -x assembler-with-cpp -undef -o $@ $<