vp8mt_alloc_temp_buffers: make prototype return void

vp8mt_alloc_temp_buffers: make prototype return void

This function was never called in a context expecting a return value,
the return value was always a constant, and the !CONFIG_MULTITHREAD
path didn't have a return statement, which caused a compiler warning.
This patch changes the function to return void instead.

Fixes issue #231

Change-Id: I9ef7f56e54418b7265026c54fc4ed5660c1418d1