Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/netmessagemaker.h

@@ -12,7 +12,7 @@
 class CNetMsgMaker
 {
 public:
-    CNetMsgMaker(int nVersionIn) : nVersion(nVersionIn){}
+    explicit CNetMsgMaker(int nVersionIn) : nVersion(nVersionIn){}
 
     template <typename... Args>
     CSerializedNetMsg Make(int nFlags, std::string sCommand, Args&&... args) const