directx_va: Fix building with latest mingw-w64 from git
Since mingw-w64 20331109d1c90c19a510408ab119a76004bc41ea (September 17th), mingw-w64 headers do provide these GUIDs. (They come from Wine as upstream, from Wine commit 6dfb84f5cbdf30c1dc0775fd4dae821ee8705ad4 from September 10th.)
This change does break building with older mingw-w64 13 git snapshots though.