xwayland: Do not list option "-eglstream" if not supported

As Xwayland is usually spawned by the Wayland server/compositor, its
command line options are not always adjustable.

Yet, if EGLStream is not supported in a given Xwayland build, the option
will do nothing (yet we must still accept it otherwise Xwayland would
refuse to run if the Wayland compositor uses it).

If Xwayland was built without support for EGLStream, there is not point
in showing the option in the help message though.

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

hw/xwayland/xwayland.c

@@ -89,7 +89,9 @@ ddxUseMsg(void)
     ErrorF("-initfd fd             add given fd as a listen socket for initialization clients\n");
     ErrorF("-listenfd fd           add given fd as a listen socket\n");
     ErrorF("-listen fd             deprecated, use \"-listenfd\" instead\n");
+#ifdef XWL_HAS_EGLSTREAM
     ErrorF("-eglstream             use eglstream backend for nvidia GPUs\n");
+#endif
     ErrorF("-shm                   use shared memory for passing buffers\n");
     ErrorF("-version               show the server version and exit\n");
 }