Add missing <stdarg.h> include for va_list

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/include/ndp.h
===================================================================
--- a/include/ndp.h
+++ b/include/ndp.h
@@ -20,6 +20,7 @@
 #ifndef _NDP_H_
 #define _NDP_H_
 
+#include <stdarg.h>
 #include <stdbool.h>
 #include <netinet/in.h>