contrib: mfx: fix linking with x86 DLL on UWP

intel_gfx_api-x86.dll exports InitialiseMediaSession and DisposeMediaSession, but libqsv_plugin.dll imports InitialiseMediaSession@12 and DisposeMediaSession@4. The name decorations are caused by the APIENTRY modifier, resulting in libqsv_plugin.dll not being able to load intel_gfx_api-x86.dll. It fails with an Entry Point Not Found error. On debug mode UWP, this will crash the CoreCLR.

Should fix LibVLCSharp issue 374.

Backport from upstream mfx_dispatch, fixed since f45c0c2.

Replaces !794 (closed)