vp8,loopfilter_filters: make some functions static
vp8,loopfilter_filters: make some functions static

quiets -Wmissing-prototypes

Change-Id: Ie5b00537f64a05e68a38dc558463691523988994

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