Fix Windows build.

While trying to resolve our CI issues, the Windows build broke with an
unused function error:

   src/test/test_switch_id.c:37:1: error: ‘unprivileged_port_range_start’
   defined but not used [-Werror=unused-function]

We solve this by moving the `#if !defined(_WIN32)` test above the
`unprivileged_port_range_start()` function defintion such that it is
included in its body.

This is an unreviewed commit.

See: tor#40275