Make definition and declaration consistent

This fixes a MSVC compiler warning.

Change-Id: Idca0815fb75127f8cd7d16311dbb50d9c80cbed8
diff --git a/av1/encoder/encodetxb.h b/av1/encoder/encodetxb.h
index c083aa5..0442cc6 100644
--- a/av1/encoder/encodetxb.h
+++ b/av1/encoder/encodetxb.h
@@ -76,9 +76,10 @@
                           int mi_row, int mi_col);
 
 void hbt_destroy();
-int av1_optimize_txb_new(const AV1_COMP *cpi, MACROBLOCK *x, int plane,
+int av1_optimize_txb_new(const struct AV1_COMP *cpi, MACROBLOCK *x, int plane,
                          int block, TX_SIZE tx_size, TX_TYPE tx_type,
-                         const TXB_CTX *txb_ctx, int *rate_cost, int sharpness);
+                         const TXB_CTX *const txb_ctx, int *rate_cost,
+                         int sharpness);
 #ifdef __cplusplus
 }
 #endif