Fix missing include

[MBkkt]

https://github.com/google/s2geometry/blob/master/src/s2/s2polygon.cc#L1293

[jmr]

I'm not sure why this is using a multi_map instead of a priority_queue, but looks good for now. Thanks.

[MBkkt]

@jmr probably because priority_queue::top return const reference and it's cannot be used with unique_ptr.

But of course exist way to avoid this issue:

1. std::vector + push_heap/pop_heap
2. const_cast

[jmr]

That would make sense, but this was written before unique_ptr was used.