tcp: Make tcp_var.h more self-contained

struct tcpcb embeds a struct osd and a struct callout. Rather than
forcing all consumers to pull in the same headers, include the headers
directly.

No functional change intended.

Reviewed by: glebius
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44685

(cherry picked from commit 1d14e88e5332cfddbec1893f6b5332f81d378d61)