vp9_encodemv.h: fix function prototype

mismatch was introduced here:
abe21e42 Removing redundant vp9_clear_system_state() call.

Change-Id: I66f945c1ce385dddfdefa9f517f295dbdd01d286

vp9/encoder/vp9_encodemv.h

@@ -26,7 +26,7 @@ void vp9_encode_mv(VP9_COMP *cpi, vp9_writer* w, const MV* mv, const MV* ref,
                    const nmv_context* mvctx, int usehp);
 
 void vp9_build_nmv_cost_table(int *mvjoint, int *mvcost[2],
-                              const nmv_context* const mvctx, int usehp);
+                              const nmv_context* mvctx, int usehp);
 
 void vp9_update_mv_count(VP9_COMP *cpi, MACROBLOCK *x, int_mv best_ref_mv[2]);