[build] Use case insensitive checks for the allowed Windows libraries

Summary: The files names in Windows are case insensitive so it makes sense to use a case insensitive check. This will fix the discrepeancy between KERNEL32.dll and kernel32.dll that we are currently facing depending the tools versions.

Test Plan: Run the windows GUIX build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18392