crypto: public_key: select CRYPTO_AKCIPHER

crypto: public_key: select CRYPTO_AKCIPHER

commit bad6a185b4d6f81d0ed2b6e4c16307969f160b95 upstream.

In some rare randconfig builds, we can end up with
ASYMMETRIC_PUBLIC_KEY_SUBTYPE enabled but CRYPTO_AKCIPHER disabled,
which fails to link because of the reference to crypto_alloc_akcipher:

crypto/built-in.o: In function `public_key_verify_signature':
:(.text+0x110e4): undefined reference to `crypto_alloc_akcipher'

This adds a Kconfig 'select' statement to ensure the dependency
is always there.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>