certs: remove unneeded -I$(srctree) option for system_certificates.o

The .incbin directive in certs/system_certificates.S includes
certs/signing_key.x509 and certs/x509_certificate_list, both of which
are generated by extract_certs, i.e. exist in $(objtree).

This option -I$(srctree) is unneeded.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>

certs/Makefile

@@ -22,9 +22,6 @@ $(eval $(call config_filename,SYSTEM_TRUSTED_KEYS))
 # GCC doesn't include .incbin files in -MD generated dependencies (PR#66871)
 $(obj)/system_certificates.o: $(obj)/x509_certificate_list
 
-# Cope with signing_key.x509 existing in $(srctree) not $(objtree)
-AFLAGS_system_certificates.o := -I$(srctree)
-
 targets += x509_certificate_list
 $(obj)/x509_certificate_list: scripts/extract-cert $(SYSTEM_TRUSTED_KEYS_SRCPREFIX)$(SYSTEM_TRUSTED_KEYS_FILENAME) FORCE
 	$(call if_changed,extract_certs,$(SYSTEM_TRUSTED_KEYS_SRCPREFIX)$(CONFIG_SYSTEM_TRUSTED_KEYS))