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/STPathSet.h

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