tcp: improve compilability

When building with DDB support, the inclusion of in_kdtrace.h
is needed. Make this explicit and don't rely on tcp_var.h to do this.
This is required for stable/14.

Fixes: a62c6b0de48a ("ddb: add optional printing of BBLog entries")
Sponsored by: Netflix, Inc.

(cherry picked from commit 4cb50d74c19c014e8099272777eb20aaf834d61c)