input_common/udp: Add missing header guard

src/input_common/udp/udp.h

@@ -2,6 +2,8 @@
 // Licensed under GPLv2 or any later version
 // Refer to the license.txt file included.
 
+#pragma once
+
 #include <memory>
 #include <unordered_map>
 #include "input_common/main.h"