Upde ffx_api.h For Cross-Platform Environment

[LocketGoma]

DllExport Error Edit.

Older Code has Error Occur For Unreal Engine's Cross-Platform Environment.

Like This.

/Myproj/Engine/Plugins/FSR4/Source/fidelityfx-sdk/Kits/FidelityFX/api/include/ffx_api.h:132:1: error: '__declspec' attributes are not enabled; use '-fdeclspec' or '-fms-extensions' to enable support for __declspec attributes
/Myproj/Engine/Plugins/FSR4/Source/fidelityfx-sdk/Kits/FidelityFX/api/include/ffx_api.h:29:23: note: expanded from macro 'FFX_API_ENTRY'
#define FFX_API_ENTRY __declspec(dllexport)