swscale: fix NEON hscale init

swscale: fix NEON hscale init

The NEON hscale function only supports X8 filter sizes and should only
be selected when these are being used. At the moment filterAlign is
set to 8 but in the future when extra NEON assembly for specific sizes is
added they will need to have checks here too.

The immediate usecase for this change is making the hscale checkasm
test easier and without NEON specific edge-cases (x86 already has these
guards).

Signed-off-by: Josh de Kock <josh@itanimul.li>