Several unused functions in `posixmodule.c` under WASI build

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Bug report

../../Modules/posixmodule.c:7883:1: warning: unused function 'warn_about_fork_with_threads' [-Wunused-function]
../../Modules/posixmodule.c:12545:1: warning: unused function 'major_minor_conv' [-Wunused-function]
../../Modules/posixmodule.c:12556:1: warning: unused function 'major_minor_check' [-Wunused-function]

Link: https://buildbot.python.org/all/#/builders/1046/builds/5439

The problem is that these helpers are defined without a #ifdef guard, while functions that use them have a conditional definition.

I propose to add the same conditions to these functions as well.

Linked PRs

• gh-120586: Fix several "unused function" warnings in posixmodule.c #120588
• [3.13] gh-120586: Fix several "unused function" warnings in posixmodule.c (GH-120588) #120616
• [3.12] gh-120586: Fix several "unused function" warnings in posixmodule.c (GH-120588) #120617

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Refs 7111d96 and e72830353216