message: fix g++9 build
diff: @@ -322,6 +322,8 @@ public:
  const BasicBlock& operator*() const;
  target_iterator& operator++(void) { ++idx; return *this; }
  bool operator==(const target_iterator &rhs) const { return idx == rhs.idx; }
  + // FIXME: remove when we drop g++9 support
  + bool operator!=(const target_iterator &rhs) const { return idx != rhs.idx; }
  };
  class it_helper {
  