certs: Fix build error when CONFIG_MODULE_SIG_KEY is PKCS#11 URI

When CONFIG_MODULE_SIG_KEY is PKCS#11 URL (pkcs11:*), signing_key.x509
fails to build:

  certs/Makefile:77: *** target pattern contains no '%'.  Stop.

Due to the typo, $(X509_DEP) contains a colon.

Fix it.

Fixes: b8c96a6b

 ("certs: simplify $(srctree)/ handling and remove config_filename macro")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>