lib: crypto: enable x509_check_for_self_signed()

When the file, x509_public_key.c, was imported from linux code in
    commit b4adf627 ("lib: crypto: add x509 parser"),
x509_check_for_self_signed() was commented out for simplicity.

Now it need be enabled in order to make pkcs7_verify_one(), which will be
imported in a later patch, functional.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>