half_btf_avx2: correct fn sig for visual studio

fixes:
formal parameter with __declspec(align('32')) won't be aligned

this is the same change that was made previously for sse4:
5bedd5dc8 idct16x16_sse4_1: correct fn sig for visual studio

Change-Id: Ib520bde439b03f81d5e84a2711ed61215debe862
1 file changed