allow for optimized versions of apply_temporal_filter
(now vp8_apply_temporal_filter_c)

the function was previously declared as static and appears to have been
inlined. with this change, that's no longer possible. performance takes
a small hit.

the declaration for vp8_cx_temp_filter_c was moved to onyx_if.c because
of a circular dependency. for rtcd, temporal_filter.h holds the
definition for the rtcd table, so it needs to be included by onyx_int.h.
however, onyx_int.h holds the definition for VP8_COMP which is needed
for the function prototype. blah.

Change-Id: I499c055fdc652ac4659c21c5a55fe10ceb7e95e3