Fix sparse warnings: non-ANSI function declarations, 0 used as pointer
diff --git a/extensions/libxt_esp.c b/extensions/libxt_esp.c
index 8811d66..adeeec7 100644
--- a/extensions/libxt_esp.c
+++ b/extensions/libxt_esp.c
@@ -20,8 +20,8 @@
 }
 
 static const struct option opts[] = {
-	{ "espspi", 1, 0, '1' },
-	{0}
+	{ "espspi", 1, NULL, '1' },
+	{ }
 };
 
 static u_int32_t