C++ fixed compile error C2678 with msvc when using std::find_if on vectors (#4262)

In Debug mode it is checked that iterator begin is less than end
therefore the operator< in class VectorIterator is needed

include/flatbuffers/flatbuffers.h

@@ -296,6 +296,10 @@ public:
     return data_ == other.data_;
   }
 
+  bool operator<(const VectorIterator &other) const {
+    return data_ < other.data_;
+  }
+
   bool operator!=(const VectorIterator &other) const {
     return data_ != other.data_;
   }