KEYS: asymmetric: Add missing dependency on CRYPTO_SIG

Make ASYMMETRIC_PUBLIC_KEY_SUBTYPE select CRYPTO_SIG to avoid build
errors like the following, which were possible with
CONFIG_ASYMMETRIC_PUBLIC_KEY_SUBTYPE=y && CONFIG_CRYPTO_SIG=n:

    ld: vmlinux.o: in function `public_key_verify_signature':
    (.text+0x306280): undefined reference to `crypto_alloc_sig'
    ld: (.text+0x306300): undefined reference to `crypto_sig_set_pubkey'
    ld: (.text+0x306324): undefined reference to `crypto_sig_verify'
    ld: (.text+0x30636c): undefined reference to `crypto_sig_set_privkey'

Fixes: 63ba4d67 ("KEYS: asymmetric: Use new crypto interface without scatterlists")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>