Fix Windows build for 0.3.5

This is a 0.3.5 backport of #40275 (closed).

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: #40275 (closed)