vp8: mark intra_prediction_down_copy inline
vp8: mark intra_prediction_down_copy inline

avoids -Wunused-function warnings when INLINE is set

Change-Id: I44d91eaa7efba7bc2427501fb9f63a93f32aaa7f

ittiamvpx/libvpx-1,Topopiccione/libvpx,mwgoldsmith/libvpx,GrokImageCompression/aom,ShiftMediaProject/libvpx,luctrudeau/aom,luctrudeau/aom,mbebenita/aom,ShiftMediaProject/libvpx,luctrudeau/aom,mbebenita/aom,mbebenita/aom,mwgoldsmith/libvpx,ittiamvpx/libvpx-1,mwgoldsmith/vpx,mbebenita/aom,GrokImageCompression/aom,ittiamvpx/libvpx-1,luctrudeau/aom,mwgoldsmith/vpx,mbebenita/aom,Topopiccione/libvpx,ittiamvpx/libvpx-1,mwgoldsmith/libvpx,mwgoldsmith/libvpx,smarter/aom,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,GrokImageCompression/aom,webmproject/libvpx,GrokImageCompression/aom,ittiamvpx/libvpx-1,mwgoldsmith/vpx,smarter/aom,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,webmproject/libvpx,smarter/aom,luctrudeau/aom,mwgoldsmith/vpx,webmproject/libvpx,mwgoldsmith/vpx,GrokImageCompression/aom,Topopiccione/libvpx,smarter/aom,luctrudeau/aom,smarter/aom,Topopiccione/libvpx,webmproject/libvpx,mbebenita/aom,mbebenita/aom,Topopiccione/libvpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,smarter/aom,GrokImageCompression/aom,webmproject/libvpx,mbebenita/aom,webmproject/libvpx,mbebenita/aom