sntrup761: use const-time helpers for memory comparison and cond move
* cipher/sntrup761.c (Ciphertexts_diff_mask): Use '_gcry_ct_not_memequal' for checking if arrays are different. (sntrup761_dec): Use '_gcry_ct_memmov_cond' for conditionally setting 'r_enc'.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>