This fixes dxva2 detection (i.e. correctly realizes that it isn't
available) for WinRT, where dxva2api.h does exist, but these definitions
are omitted (when targeting the API subsets).

Ideally we should rather check for e.g. DXVA2_ConfigPictureDecode,
but configure might fail to find that definition due to _WIN32_WINNT
not being set to the right value during configure. (libavcodec/dxva2.h
manually overrides the _WIN32_WINNT define.)

This allows removing hardcoded --disable-dxva2 from such build
configurations.
Signed-off-by: Martin Storsjö <martin@martin.st>