tools: net: add <optional> header

The constant std::nullopt is used but the header was not included.

Change-Id: Ibb79f7f0b9944425202695b8c5d4e1d191b72a69
Signed-off-by: Benjamin Fair <benjaminfair@google.com>
diff --git a/tools/net.cpp b/tools/net.cpp
index 4d2ce13..9027862 100644
--- a/tools/net.cpp
+++ b/tools/net.cpp
@@ -30,6 +30,7 @@
 #include <cstring>
 #include <ipmiblob/blob_errors.hpp>
 #include <memory>
+#include <optional>
 #include <stdplus/handle/managed.hpp>
 #include <string>
 #include <vector>