certs: make system keyring depend on built-in x509 parser

Commit e9088629 ("certs: make system keyring depend on x509 parser")
is not the right fix because x509_load_certificate_list() can be modular.

The combination of CONFIG_SYSTEM_TRUSTED_KEYRING=y and
CONFIG_X509_CERTIFICATE_PARSER=m still results in the following error:

    LD      .tmp_vmlinux.kallsyms1
  ld: certs/system_keyring.o: in function `load_system_certificate_list':
  system_keyring.c:(.init.text+0x8c): undefined reference to `x509_load_certificate_list'
  make: *** [Makefile:1169: vmlinux] Error 1

Fixes: e9088629 ("certs: make system keyring depend on x509 parser")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Adam Borowski <kilobyte@angband.pl>

certs/Kconfig

@@ -43,7 +43,7 @@ config SYSTEM_TRUSTED_KEYRING
 	bool "Provide system-wide ring of trusted keys"
 	depends on KEYS
 	depends on ASYMMETRIC_KEY_TYPE
-	depends on X509_CERTIFICATE_PARSER
+	depends on X509_CERTIFICATE_PARSER = y
 	help
 	  Provide a system keyring to which trusted keys can be added.  Keys in
 	  the keyring are considered to be trusted.  Keys may be added at will