fix(thorvg): fix windows build break

[liamHowatt]

Description of the feature or fix

In response to #5946

Error when building thorvg on Windows. Defined functions were being marked with __declspec(dllimport).

lvgl/src/libs/thorvg/thorvg_capi.h

Lines 35 to 40 in fea8a97

	#ifdef _WIN32
	#if TVG_BUILD
	#define TVG_API __declspec(dllexport)
	#else
	#define TVG_API __declspec(dllimport)
	#endif

This fix tries to leave the thorvg header untouched except for defining TVG_BUILD at the top so it's easier to update thorvg from the upstream. #4691 (comment)

Would have been found by the eventual Windows CI #5904

Also remove duplicate guard.

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[kisvegabor]

Having a Windows build test would be really great.. (#5904)

@liamHowatt do you feel the power to add a Windows build test to the CI?

[liamHowatt]

@liamHowatt do you feel the power to add a Windows build test to the CI?

After some brief exploration I think I have the power to do it. It was easy enough to use the windows-latest runner. It comes with MinGW GCC and the chocolatey package manager and many more things we can use to make it happen.

Here's everything that comes with the windows runner: https://github.com/actions/runner-images/blob/win22/20240322.1/images/windows/Windows2022-Readme.md

My branch is https://github.com/liamHowatt/lvgl/tree/feat/windows-ci

[liamHowatt]

Oh, hang on, I'll verify MinGW GCC treats everything the same as MSVC. I know that MinGW GCC defines _WIN32. But I'll need to see about #5873. No matter either way. Visual Studio is also installed in the runner.

[kisvegabor]

I think MSVC is the real issue and focus on that.

[liamHowatt]

See my comment in the Windows CI build issue and the PR that builds on Windows with GCC for now.