vp9_encodeframe: make scale_part_thresh_sumdiff static
vp9_encodeframe: make scale_part_thresh_sumdiff static

quiets -Wmissing-prototypes

Change-Id: I696223d75860edba13c6b6f38c1f8db353a6f812

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