x509: select CONFIG_CRYPTO_LIB_SHA256

The x509 public key code gained a dependency on the sha256 hash
implementation, causing a rare link time failure in randconfig
builds:

arm-linux-gnueabi-ld: crypto/asymmetric_keys/x509_public_key.o: in function `x509_get_sig_params':
x509_public_key.c:(.text.x509_get_sig_params+0x12): undefined reference to `sha256'
arm-linux-gnueabi-ld: (sha256): Unknown destination type (ARM/Thumb) in crypto/asymmetric_keys/x509_public_key.o
x509_public_key.c:(.text.x509_get_sig_params+0x12): dangerous relocation: unsupported relocation

Select the necessary library code from Kconfig.

Fixes: 2c62068ac86b ("x509: Separately calculate sha256 for blacklist")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>