Fix return type.

[a-balashov]

Hello! I get an error due to include:

#include <contrib/libs/libosmium/include/osmium/relations/relations_manager.hpp>

I use Ubuntu 20.04.1 LTS, clang version 12.0.1, libosmium version 2.17.0.

In file included from /contrib/libs/libosmium/include/osmium/relations/relations_manager.hpp:45:
/contrib/libs/libosmium/include/osmium/relations/members_database.hpp:162:55: error: 'osmium::iterator_range<osmium::relations::MembersDatabaseCommon::element *>::iterator' (aka 'osmium::relations::MembersDatabaseCommon::element *') is not a class, namespace, or enumeration
            static typename iterator_range<iterator>::iterator::difference_type count_not_removed(const iterator_range<iterator>& range) noexcept {
                                                      ^
1 error generated.
Failed

So I make PR to fix this problem.