cipher:pubkey: Fix non-use of flexible array member.
* cipher/pubkey.c (struct pk_single_data): Use 1 as the size. (_gcry_pk_single_data_push): Use offsetof.
Backport master commit of:
c160e1a85f8295e388de7a0b09a351bc271f0d95
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>