gpg: Replace use of PRIu64 in log_debug

* g10/cipher-aead.c (do_flush): Use %llu and a cast.
* g10/decrypt-data.c (aead_underflow): Ditto.

We don't use the system's printf but the one implemented by
us (gpgrt's estream-printf) thus the PRIu64 may or may not be correct.
We can't do much about the -Wformat errors due to our different
implementation.

• Fixes-commit: b2cedc108d5cabb07f496c31c11c9cba6f328f76