Readd a missed prototype for the WelsProcessingSampleSad8x8_neon function

The common WelsSampleSad8x8_neon function has different alignment
requirements than WelsProcessingSampleSad8x8_neon - until it has
been sorted out that the common version can be used in the processing
lib, the separate version for processing is used.

This fixes building with neon optimizations enabled.
Merge pull request #578 from mstorsjo/fix-arm-build

Readd a missed prototype for the WelsProcessingSampleSad8x8_neon function