includes: avoid installing if_wg.h twice

if_wg.h was installed via dev/wg in LSUBDIRS and also explicitly. We
want to install only wg/if_wg.h not the other headers, so add dev/wg to
the skip list in the copies and symlinks targets.

PR: 271266
Reviewed by: kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40031