fwd_txfm_msa.c: add missing vpx_dsp_rtcd.h
fwd_txfm_msa.c: add missing vpx_dsp_rtcd.h

+ only expose compatible functions in high-bitdepth build

quiets -Wmissing-prototypes warnings

Change-Id: I8ef7db08a34c5c54b5cde6e732c0d70f4287c89a

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