gpg: Fix for -Wformat when using uint64_t.

* g10/cipher-aead.c (do_flush): Use PRIu64.
* g10/decrypt-data.c (aead_underflow): Likewise.

Even among LP64 data model machines, uint64_t type may differ;
unsigned long or unsigned long long.
Only portable way is use of PRIu64.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>