The struct definitions in dxva.h, which are necessary in order to
actually use d3d11va, are hidden when WINAPI_FAMILY targets Windows Phone
or WindowsRT.

Building with WINAPI_FAMILY=WINAPI_FAMILY_DESKTOP_APP is disallowed
when targeting ARM. ("Compiling Desktop applications for the ARM
platform is not supported.") So we set _CRT_BUILD_DESKTOP_APP to 0
to tell the runtime not to detect some issues with this mismatching.

The same tweaks to detect if the API is available is done in dxva2_internal.h
when compiling each DXVA2/D3D11VA decoders.
Signed-off-by: Martin Storsjö <martin@martin.st>