xwayland: Add help entry for -shm

The command line options "-shm" is used to instruct Xwayland to prefer
shared-memory for passing buffers to the Wayland server, rather than
using glamor and DRI3.

The option was there from the beginning, yet not documented in the
"-help" message.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Martin Peres <martin.peres@mupuf.org>

hw/xwayland/xwayland.c

@@ -90,6 +90,7 @@ ddxUseMsg(void)
     ErrorF("-listenfd fd           add given fd as a listen socket\n");
     ErrorF("-listen fd             deprecated, use \"-listenfd\" instead\n");
     ErrorF("-eglstream             use eglstream backend for nvidia GPUs\n");
+    ErrorF("-shm                   use shared memory for passing buffers\n");
     ErrorF("-version               show the server version and exit\n");
 }