make the borders red borders for urgent clients

iirc this is the same behavior of dwm

config.def.h

@@ -9,6 +9,7 @@ static const int bypass_surface_visibility = 0;  /* 1 means idle inhibitors will
 static const unsigned int borderpx         = 1;  /* border pixel of windows */
 static const float bordercolor[]           = COLOR(0x444444ff);
 static const float focuscolor[]            = COLOR(0x005577ff);
+static const float urgentcolor[]           = COLOR(0xff0000ff);
 /* To conform the xdg-protocol, set the alpha to zero to restore the old behavior */
 static const float fullscreen_bg[]         = {0.1, 0.1, 0.1, 1.0}; /* You can also use glsl colors */