fix: replace std::for_each_n for gcc 8 build failure

[tearfur]

Our NetBSD CI over at the Transmission project found that std::for_each_n is not supported by GCC 8 for whatever reason.

The solution in this PR is already tested to build successfully.

In file included from ../../third-party/small/include/small/map.hpp:11,
                 from ../../libtransmission/announce-list.cc:11:
../../third-party/small/include/small/vector.hpp: In member function 'constexpr small::vector<T, N, Allocator, AllowHeap, SizeType, GrowthFactor>::iterator small::vector<T, N, Allocator, AllowHeap, SizeType, GrowthFactor>::erase(small::vector<T, N, Allocator, AllowHeap, SizeType, GrowthFactor>::const_iterator, small::vector<T, N, Allocator, AllowHeap, SizeType, GrowthFactor>::const_iterator)':
../../third-party/small/include/small/vector.hpp:1090:26: error: 'for_each_n' is not a member of 'std'
                     std::for_each_n(
                          ^~~~~~~~~~
../../third-party/small/include/small/vector.hpp:1090:26: note: suggested alternative: 'for_each'
                     std::for_each_n(
                          ^~~~~~~~~~
                          for_each
Process exited with code 1

[tearfur]

CI failure is most likely unrelated to the change. We've been seeing segfaults in Ubuntu CI jobs from non-Cpp PRs.

[alandefreitas]

Yes. It should be unrelated.