vp9_block.h: rename diff struct to Diff

This matches the style guide and fixes some -Wshadow warnings related to
variables with the same name. Something similar was done in libaom in:
863b04994b Fix warnings reported by -Wshadow: Part2: av1 directory

Bug: webm:1793
Change-Id: I4df1bbc8d079a3174d75f0d35d54c200ffdbb677
4 files changed