fwd_dct32x32 avx2: resolve missing declarations
fwd_dct32x32 avx2: resolve missing declarations

BUG=webm:1584

Change-Id: Iaba854952534a95e710a985acfcab46e093872c2

webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx