libvpx: cherry pick "register" removal

remove "register" keyword

This has been deprecated for a long time. c++17 is trying to recover the name.

cherry picked from libvpx:
17004c71bcd7ae7e6a65bd5b8e756fb2311accdc

Test: none

Change-Id: Ic839e9cd25c5f989737a823fd1ba54f6217fdd17
3 files changed