sstables/compaction: Fix compilation error with GCC 4.9.2
sstables/compaction: Fix compilation error with GCC 4.9.2

I am sure it's a compiler issue but I am not ready to give up and
upgrade just yet:

  sstables/compaction.cc:307:55: error: converting to ‘std::unordered_map<int, long int>’ from initializer list would use explicit constructor ‘std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::unordered_map(std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::size_type, const hasher&, const key_equal&, const allocator_type&) [with _Key = int; _Tp = long int; _Hash = std::hash<int>; _Pred = std::equal_to<int>; _Alloc = std::allocator<std::pair<const int, long int> >; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::size_type = long unsigned int; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::hasher = std::hash<int>; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::key_equal = std::equal_to<int>; std::unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>::allocator_type = std::allocator<std::pair<const int, long int> >]’
                 stats->start_size, stats->end_size, {});

scylladb/scylla,avikivity/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,kjniemi/scylla,raphaelsc/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,kjniemi/scylla