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: Masahiro Yamada <masahiroy@kernel.org>