ng_netflow v9: fix template re-announcement using packet count

Increment fe->sent_packets after export9_send().
Previously, NetFlow v9 templates were only re-announced based on
time, ignoring the packet count parameter (templ_packets).

PR: 270083
Reviewed by: glebius pouria
Approved by: glebius (mentor)
MFC after: 1 week
Sponsored by: Subcarpathian BSD User Group

(cherry picked from commit d836dae9f0d888d441234ea11e3cd91614c86aa2)