Revert "Remove Wextra warnings from vp9_sad.c"

This reverts commit 7ab9a958

Nightly test http://build.webmproject.org/jenkins/view/libvpx-nightly-tests/job/libvpx%20unit%20tests%20(valgrind-2)/arch=x86_64-linux-gcc,filter=-*VP8*:*Large.*/276/console

Failed 

This patch did not address all the assembly issues 
some of the vp8 assembly counts on 5 arguments being passed in to this function:   

one example : vp8_sad8x16_wmt

Please address or split this into vp9 and vp8 patches.

Change-Id: I78afcc171649894f887bb8ee3c66de24aaddc7ca