Add const qualifier for function pointer table

const qualifier is required to avoid compiler warning
if model_rd.h is included in multiple files.

Change-Id: I0bf76c557388cf38a732d446780ca8a4e9ff7ed4
diff --git a/av1/encoder/model_rd.h b/av1/encoder/model_rd.h
index 53c30cc..9fc96fc 100644
--- a/av1/encoder/model_rd.h
+++ b/av1/encoder/model_rd.h
@@ -249,11 +249,11 @@
 
 enum { MODELRD_LEGACY, MODELRD_CURVFIT, MODELRD_TYPES } UENUM1BYTE(ModelRdType);
 
-static model_rd_for_sb_type model_rd_sb_fn[MODELRD_TYPES] = {
+static const model_rd_for_sb_type model_rd_sb_fn[MODELRD_TYPES] = {
   model_rd_for_sb, model_rd_for_sb_with_curvfit
 };
 
-static model_rd_from_sse_type model_rd_sse_fn[MODELRD_TYPES] = {
+static const model_rd_from_sse_type model_rd_sse_fn[MODELRD_TYPES] = {
   model_rd_from_sse, model_rd_with_curvfit
 };