COPYING: add author to HPND-sell-MIT-disclaimer-xserver

the license notice comes from these files:
- dix/registry.c
- Xext/xselinuxint.h
- Xext/xselinux_label.c
- Xext/xselinux_hooks.c
- Xext/xselinux_ext.c
- Xext/xselinux.h
- Xext/xacestr.h
- Xext/xace.h
- Xext/xace.c

while this is not like the standard `Copyright (C)` lines I think it
still belongs to the license, even if just to make it more clearly
separate from the MIT license above

(cherry picked from commit fff0023339)

Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2151>

COPYING

@@ -1783,6 +1783,8 @@ OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
 USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 
+Author: Eamon Walsh <ewalsh@tycho.nsa.gov>
+
 Permission to use, copy, modify, distribute, and sell this software and its
 documentation for any purpose is hereby granted without fee, provided that
 this permission notice appear in supporting documentation.  This permission