certs: include certs/signing_key.x509 unconditionally

I do not see much sense in the #if conditional in system_certificates.S;
even if the condition is true, there exists no signing key when
CONFIG_MODULE_SIG_KEY="".

So, certs/Makefile generates empty certs/signing_key.x509 in such a
case. We can always do this, irrespective of CONFIG_MODULE_SIG or
(CONFIG_IMA_APPRAISE_MODSIG && CONFIG_MODULES).

We only need to check CONFIG_MODULE_SIG_KEY, then both *.S and Makefile
will become much simpler.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>