tcp: make tcp_lro_flush() static

tcp_lro_flush() is not used anymore outside of tcp_lro.c. Therefore
make it static.

Reviewed by: rscheff, glebius, Peter Lei
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46435

(cherry picked from commit e06cf0fc5dd626c34acdef308b696b4995371a4b)