libweston: Silence tautological-compare warning

libweston/compositor.c:5023:14: warning: comparison of unsigned enum
expression < 0 is always false [-Wtautological-compare]
        if (backend < 0 || backend >= ARRAY_LENGTH(backend_map))
                    ~~~~~~~ ^ ~

Warning produced by Clang 3.8.

Signed-off-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Giulio Camuffo <giuliocamuffo@gmail.com>

libweston/compositor.c

@@ -5093,7 +5093,7 @@ weston_compositor_load_backend(struct weston_compositor *compositor,
 	int (*backend_init)(struct weston_compositor *c,
 			    struct weston_backend_config *config_base);
 
-	if (backend < 0 || backend >= ARRAY_LENGTH(backend_map))
+	if (backend >= ARRAY_LENGTH(backend_map))
 		return -1;
 
 	backend_init = weston_load_module(backend_map[backend], "backend_init");