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;

src/ripple/protocol/STObject.h

@@ -98,6 +98,7 @@ private:
             typename T::value_type;
 
     public:
+        ValueProxy(ValueProxy const&) = default;
         ValueProxy& operator= (ValueProxy const&) = delete;
 
         template <class U>
@@ -125,6 +126,7 @@ private:
             typename std::decay<value_type>::type>;
 
     public:
+        OptionalProxy(OptionalProxy const&) = default;
         OptionalProxy& operator= (OptionalProxy const&) = delete;
 
         /** Returns `true` if the field is set.

src/ripple/protocol/STPathSet.h

@@ -118,6 +118,7 @@ public:
     }
 
     STPathElement(STPathElement const&) = default;
+    STPathElement& operator=(STPathElement const&) = default;
 
     int
     getNodeType () const