crypto: certs: fix FIPS selftest dependency

The selftest code is built into the x509_key_parser module, and depends
on the pkcs7_message_parser module, which in turn has a dependency on
the key parser, creating a dependency loop and a resulting link
failure when the pkcs7 code is a loadable module:

ld: crypto/asymmetric_keys/selftest.o: in function `fips_signature_selftest':
crypto/asymmetric_keys/selftest.c:205: undefined reference to `pkcs7_parse_message'
ld: crypto/asymmetric_keys/selftest.c:209: undefined reference to `pkcs7_supply_detached_data'
ld: crypto/asymmetric_keys/selftest.c:211: undefined reference to `pkcs7_verify'
ld: crypto/asymmetric_keys/selftest.c:215: undefined reference to `pkcs7_validate_trust'
ld: crypto/asymmetric_keys/selftest.c:219: undefined reference to `pkcs7_free_message'

Avoid this by only allowing the selftest to be enabled when either
both parts are loadable modules, or both are built-in.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>