Include opt.h so that LWIP_ERROR works correctly
diff --git a/src/include/lwip/debug.h b/src/include/lwip/debug.h
index d8359ea..0fe0413 100644
--- a/src/include/lwip/debug.h
+++ b/src/include/lwip/debug.h
@@ -33,6 +33,7 @@
 #define __LWIP_DEBUG_H__
 
 #include "lwip/arch.h"
+#include "lwip/opt.h"
 
 /** lower two bits indicate debug level
  * - 0 all