Port: fix format warnings on Cygwin

On Cygwin, be64toh() returns a "long long unsigned int" while the
format specifier PRIu64 expects a "long unsigned int". Both types
are 64bits integers, just cast the result to uint64_t to silence
the warnings.

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