Fix building shared libraries on Windows using MinGW64

[SavaLione]

Summary

Hi,
There was an issue to building MaterialX on a Windows system using the MinGW64 compiler:

• Windows MinGW shared build broken #2948

This PR fixes this compilation issue, and the dynamical libraries are now successfully build.

The referenced issue suggests the addition of new GitHub CI entries.
However, this PR does not include them.
I can work on the CI implementation if it is still required and the additional information is provided:

• How is it acceptable get the MinGW binaries (e.g., whether binaries provided by Stephan T. Lavavej are acceptable)
• Which additional tools may a container download and use (is it possible to use bash or ninja?)
• What is the preferred way to cache these binaries?

Tests

Tests were successfully compiled and executed.
All 94 tests cases passed (39629 assertions).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: SavaLione / name: Savelii Pototskii (c64c955)

[jstone-lucasfilm]

Thanks for this proposal, @SavaLione! Can you add a comment on the GitHub Issue that you're tackling, so that we can assign it to you? Additionally, see the instructions at #3075 (comment) on how to resolve the CLA authorization warnings, so that we can begin reviewing this PR.

[SavaLione]

The CLA was signed via EasyCLA, a comment was also added to the issue

[jstone-lucasfilm]

Apologies, @SavaLione, another developer requested this one first, but feel free to request any other unclaimed issue for Dev Days.

[SavaLione]

Yeah, sure, I should've checked the full issue conversation earlier.
It still may be useful to the person who initially requested this issue first.
And there a new workflow has to be added, so the current PR is not fully covering all requirements from the issue.

[jstone-lucasfilm]

Thanks for understanding, @SavaLione, and indeed the other developer may find this work useful for their task. I've added a few more Good First Issues to our list for Dev Days, if you'd like to take on another task for the event tomorrow.

[cod1r]

I think I will add to this PR and finish out the CI stuff if that's ok.

I don't want to throw away mister Sava's good work here :)

[jstone-lucasfilm]

Much appreciated, @cod1r, and I think that's a great path forward.

[jstone-lucasfilm]

This change looks correct to me, thanks @SavaLione, and let's move forward with the merge, so that the next steps proposed in #3081 can be validated.