fix mem leak in cleanup()

maybe leak isn't the best word, given that the object lives for the
entire duration of the program's lifetime.

however, all elements of scheme are free-ed, can't think of any reason
why scheme itself should be an exception.

dwm.c

@@ -487,6 +487,7 @@ cleanup(void)
 		drw_cur_free(drw, cursor[i]);
 	for (i = 0; i < LENGTH(colors); i++)
 		free(scheme[i]);
+	free(scheme);
 	XDestroyWindow(dpy, wmcheckwin);
 	drw_free(drw);
 	XSync(dpy, False);