vp8: mark intra_prediction_down_copy inline

avoids -Wunused-function warnings when INLINE is set

Change-Id: I44d91eaa7efba7bc2427501fb9f63a93f32aaa7f

vp8/common/reconintra4x4.h

diff --git a/vp8/common/reconintra4x4.h b/vp8/common/reconintra4x4.h
index 869841e..5dc5d13 100644
--- a/vp8/common/reconintra4x4.h
+++ b/vp8/common/reconintra4x4.h
@@ -17,8 +17,8 @@
 extern "C" {
 #endif
 
-static void intra_prediction_down_copy(MACROBLOCKD *xd,
-                                       unsigned char *above_right_src)
+static INLINE void intra_prediction_down_copy(MACROBLOCKD *xd,
+                                              unsigned char *above_right_src)
 {
     int dst_stride = xd->dst.y_stride;
     unsigned char *above_right_dst = xd->dst.y_buffer - dst_stride + 16;