vp8 machine setup: mark unused variable
vp8 machine setup: mark unused variable

When building without multithreading and for a non-arm, non-x86 system,
ctx is unused.

Cleans up -Wextra warning:
unused parameter ‘ctx’ [-Werror=unused-parameter]

Change-Id: Ifddff89d2ebd45f7d71e3d415a8f2415dd818957

mwgoldsmith/libvpx,Topopiccione/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,Topopiccione/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,ShiftMediaProject/libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,Topopiccione/libvpx