Remove use of deprecated behavior involving copy members

*  If any of the destructor, copy assignment or copy constructor
   are user-declared, both copy members should be user-declared,
   otherwise the compiler-generation of them is deprecated.

src/ripple/protocol/STBase.h

@@ -67,6 +67,7 @@ public:
 
     virtual ~STBase() = default;
 
+    STBase(const STBase& t) = default;
     STBase& operator= (const STBase& t);
 
     bool operator== (const STBase& t) const;