crypto: drbg - should select CTR

if CRYPTO_DRBG_CTR is builtin and CTR is module, allocating such algo
will fail.
DRBG: could not allocate CTR cipher TFM handle: ctr(aes)
alg: drbg: Failed to reset rng
alg: drbg: Test 0 failed for drbg_pr_ctr_aes128
DRBG: could not allocate CTR cipher TFM handle: ctr(aes)
alg: drbg: Failed to reset rng
alg: drbg: Test 0 failed for drbg_nopr_ctr_aes128
DRBG: could not allocate CTR cipher TFM handle: ctr(aes)
alg: drbg: Failed to reset rng
alg: drbg: Test 0 failed for drbg_nopr_ctr_aes192
DRBG: could not allocate CTR cipher TFM handle: ctr(aes)
alg: drbg: Failed to reset rng
alg: drbg: Test 0 failed for drbg_nopr_ctr_aes256

So let's select CTR instead of just depend on it.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>