COPYING: add missing paragraph to SGI-B-2.0

all files that this license applies to include the last paragraph:
- glx/xfont.c
- glx/unpack.h
- glx/singlesize.h
- glx/singlesize.c
- glx/singlepixswap.c
- glx/singlepix.c
- glx/single2swap.c
- glx/single2.c
- glx/rensize.c
- glx/renderpixswap.c
- glx/renderpix.c
- glx/render2swap.c
- glx/render2.c
- glx/glxutil.h
- glx/glxdrawable.h
- glx/glxserver.h
- glx/glxcontext.h
- glx/glxscreens.h
- glx/glxscreens.c
- glx/glxcmdsswap.c
- glx/glxext.h
- glx/glxcmds.c
- glx/glxext.c

the paragraph was likely accidentally missed when the license changed
from SGI-B-1.0 to SGI-B-2.0 in ed9aecf851

(cherry picked from commit 6105e5b133)

Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2146>

COPYING

@@ -314,6 +314,11 @@ WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF
 OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 
+Except as contained in this notice, the name of Silicon Graphics, Inc.
+shall not be used in advertising or otherwise to promote the sale, use or
+other dealings in this Software without prior written authorization from
+Silicon Graphics, Inc.
+
 
 Copyright (c) 1994, 1995  Hewlett-Packard Company