libpfct: be consistent with u_int vs. uint

Always use uint64_t over u_int64_t, for the sake of consistency.

No functional change.

MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 7bb3c927f7d9c48b356b4d20907b813f9b83273b)