Fix mismatched tag warning for MapByIndex (#658)

- [ X] Keep your PR rebased to master.
- [ X] Keep your PR under 200 lines of code and address a single concern.
- [ X] Add unit test(s) and documentation (these do not count toward your 200 lines).
- [ X] Adhere to the [Google C++ Style Guide](https://google.github.io/styleguide/cppguide.html).
- [ X] Run `clang-format -style=Google` on your code (this alone is not enough to ensure you've followed the style guide).
- [ X] Run `ninja test` or `catkin_make_isolated --install --use-ninja --pkg cartographer --make-args test` as appropriate.

cartographer/mapping/id.h

@@ -102,7 +102,7 @@ class Range {
 template <typename IdType, typename DataType>
 class MapById {
  private:
-  class MapByIndex;
+  struct MapByIndex;
 
  public:
   struct IdDataReference {