Mark AVX2 intra filter functions as static.
Mark AVX2 intra filter functions as static.

Marks functions filter_4x4_avx2, filter_16x16_avx2 and filter_NxN_avx2
static as they are not used outside strategies/avx2/intra-avx2.

ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar,ultravideo/kvazaar