avoid negative array subscript warnings

The mv_ref and sub_mv_ref token encodings are indexed from NEARESTMV
and LEFT4X4, respectively, rather than being zero-based like the
other token encodings.

Change-Id: I3699c3f84111209ecfb91097c4b900773e9a3ad5

vp8/common/treecoder.h

@@ -54,6 +54,8 @@ typedef const struct vp8_token_struct
 /* Construct encoding array from tree. */
 
 void vp8_tokens_from_tree(struct vp8_token_struct *, vp8_tree);
+void vp8_tokens_from_tree_offset(struct vp8_token_struct *, vp8_tree,
+                                 int offset);
 
 
 /* Convert array of token occurrence counts into a table of probabilities