Ok, I can reproduce the issue now. The latest MinGW-w64 release (2.0.4) includes changes to dxva2api.h which borks the FFmpeg build. FFmpeg have already updated their sources. So I guess you have two options:
1. Add the following line to libavcodec/dxva2_internal.h:
#include <dxva.h>2. Revert to niXman's 4.7.1-rev0 build, which includes MinGW-w64 2.0.3.