wg(4): Add authors of the wg(4) driver to the manual page

Glen (@gjb) noticed that I am haven't mentioned the authors of the
WireGuard device driver in the manual page.

This is commit addressed this.

Reviewed by: gjb, brueffer
Differential Revision: https://reviews.freebsd.org/D28464

(cherry picked from commit c1b1354789bb15713944f9e6941266de4cb47772)