gpg: Use GCRY_KDF_ONESTEP_KDF with newer libgcrypt in future.

* g10/ecdh.c (derive_kek): Use GCRY_KDF_ONESTEP_KDF.

This change is not yet enabled. We will be able to use the code when
we update NEED_LIBGCRYPT_VERSION to 1.11.0. Before the update, gpg
compiled with libgcrypt 1.11.0 can't work with older libgcrypt
runtime.

• GnuPG-bug-id: T5964
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>