Mk/Uses: Add USES+=xorg when including xorg.mk

Add USES+=xorg to the places where MK/Uses/xorg.mk is included from other
files in the Uses infrastructure. This is done to silence an erroneous
warning about USES=xorg not being set when using USES=gl, motif or xorg-cat,
even when it's included from the framework.

This is a different proposal than what was originally suggested in pr 241627
and https://reviews.freebsd.org/D22210 .

PR: 238988, 241627
Discussed with: mat
MFH: 2019Q4
Differential Revision: https://reviews.freebsd.org/D22210