vp9_dct_sse2: make some functions static
vp9_dct_sse2: make some functions static

silences missing prototype warnings

Change-Id: I773b6a6b5bd7c57db18c3b17c519534f80e131de

GrokImageCompression/aom,thdav/aom,luctrudeau/aom,ShiftMediaProject/libvpx,running770/libvpx,Topopiccione/libvpx,luctrudeau/aom,stewnorriss/libvpx,mwgoldsmith/libvpx,VTCSecureLLC/libvpx,smarter/aom,VTCSecureLLC/libvpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,jmvalin/aom,mwgoldsmith/libvpx,mwgoldsmith/libvpx,shyamalschandra/libvpx,mwgoldsmith/vpx,thdav/aom,smarter/aom,shacklettbp/aom,liqianggao/libvpx,webmproject/libvpx,GrokImageCompression/aom,openpeer/libvpx_new,mwgoldsmith/libvpx,jmvalin/aom,openpeer/libvpx_new,GrokImageCompression/aom,ShiftMediaProject/libvpx,shacklettbp/aom,VTCSecureLLC/libvpx,luctrudeau/aom,shyamalschandra/libvpx,luctrudeau/aom,mbebenita/aom,zofuthan/libvpx,jmvalin/aom,goodleixiao/vpx,smarter/aom,zofuthan/libvpx,running770/libvpx,felipebetancur/libvpx,luctrudeau/aom,liqianggao/libvpx,shyamalschandra/libvpx,shyamalschandra/libvpx,ittiamvpx/libvpx-1,GrokImageCompression/aom,Topopiccione/libvpx,mbebenita/aom,openpeer/libvpx_new,GrokImageCompression/aom,mbebenita/aom,webmproject/libvpx,jmvalin/aom,VTCSecureLLC/libvpx,goodleixiao/vpx,webmproject/libvpx,mbebenita/aom,ittiamvpx/libvpx-1,shacklettbp/aom,felipebetancur/libvpx,mwgoldsmith/vpx,ittiamvpx/libvpx-1,Topopiccione/libvpx,mbebenita/aom,stewnorriss/libvpx,ShiftMediaProject/libvpx,shacklettbp/aom,Topopiccione/libvpx,running770/libvpx,ShiftMediaProject/libvpx,zofuthan/libvpx,mwgoldsmith/libvpx,mbebenita/aom,thdav/aom,mwgoldsmith/vpx,felipebetancur/libvpx,zofuthan/libvpx,smarter/aom,shacklettbp/aom,smarter/aom,webmproject/libvpx,stewnorriss/libvpx,jmvalin/aom,liqianggao/libvpx,openpeer/libvpx_new,goodleixiao/vpx,goodleixiao/vpx,felipebetancur/libvpx,ittiamvpx/libvpx-1,jmvalin/aom,openpeer/libvpx_new,zofuthan/libvpx,stewnorriss/libvpx,Topopiccione/libvpx,webmproject/libvpx,thdav/aom,running770/libvpx,mwgoldsmith/libvpx,mbebenita/aom,thdav/aom,thdav/aom,luctrudeau/aom,ittiamvpx/libvpx-1,openpeer/libvpx_new,felipebetancur/libvpx,running770/libvpx,zofuthan/libvpx,shyamalschandra/libvpx,felipebetancur/libvpx,stewnorriss/libvpx,GrokImageCompression/aom,goodleixiao/vpx,mbebenita/aom,shacklettbp/aom,shyamalschandra/libvpx,VTCSecureLLC/libvpx,liqianggao/libvpx,stewnorriss/libvpx,liqianggao/libvpx,smarter/aom,running770/libvpx,goodleixiao/vpx,liqianggao/libvpx,mwgoldsmith/vpx,webmproject/libvpx,VTCSecureLLC/libvpx,ittiamvpx/libvpx-1,mbebenita/aom,Topopiccione/libvpx,mwgoldsmith/vpx