Fix -Wformat errors in pfctl on 32-bit architectures

Use PRIu64 to printf(3) uint64_t quantities, otherwise this will result
in "error: format specifies type 'unsigned long' but the argument has
type 'uint64_t' (aka 'unsigned long long') [-Werror,-Wformat]" on 32-bit
architectures.

Fixes: 80078d9d38fd
MFC after: 1 week