fwd_txfm_msa.c: correct vpx_fdct8x8_1_msa prototype
fwd_txfm_msa.c: correct vpx_fdct8x8_1_msa prototype

this makes the function compatible with high-bitdepth and fixes test
failures since:
5ac88162b partial fdct test

Change-Id: Ib630694608237f0c515948942e05dbea259ba338

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