vp9_rtcd_defs: fix vp9_avg_8x8 declaration

vp9_avg_8x8 does not depend on x86inc, fixes 32-bit OS X build

Change-Id: I709b874ea84bf57c8cdb5ac7d43eecc6b8c1a2dd
set_vt_partitioning: fix type conversion warning

double -> int64
+ make threshold_multiplier an int

Change-Id: I6d3607fdf13d670f57c9d9b04a80acb2be1346a0
Merge "set_vt_partitioning: fix type conversion warning"