opt_ref_mv: Fix build when disabled.
opt_ref_mv: Fix build when disabled.

Change-Id: I33db29b271eba902af73931a1ae139489621fcfe

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom