Port: cast result of be64toh() to uint64_t for print format

The type of the return value of be64toh() can vary from unsigned long to
unsigned long long across platforms, cast it to the longer type before
printing.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>