crypto: public_key: select CRYPTO_AKCIPHER

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.

Cc: <stable@vger.kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

crypto/asymmetric_keys/Kconfig

@@ -13,6 +13,7 @@ config ASYMMETRIC_PUBLIC_KEY_SUBTYPE
 	tristate "Asymmetric public-key crypto algorithm subtype"
 	select MPILIB
 	select CRYPTO_HASH_INFO
+	select CRYPTO_AKCIPHER
 	help
 	  This option provides support for asymmetric public key type handling.
 	  If signature generation and/or verification are to be used,