crypto: jitterentropy - always select CRYPTO_RNG

When building the jitterentropy driver by itself, we get a link error
when CRYPTO_RNG is not enabled as well:

crypto/built-in.o: In function `jent_mod_init':
jitterentropy-kcapi.c:(.init.text+0x98): undefined reference to `crypto_register_rng'
crypto/built-in.o: In function `jent_mod_exit':
jitterentropy-kcapi.c:(.exit.text+0x60): undefined reference to `crypto_unregister_rng'

This adds a 'select CRYPTO_RNG' to CRYPTO_JITTERENTROPY to ensure the API
is always there when it's used, not just when DRBG is also enabled.
CRYPTO_DRBG would set it implicitly through CRYPTO_JITTERENTROPY now,
but this leaves it in place to make it explicit what the driver does.

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