Add buf_ans.h to the Makefile.

Change-Id: I6339912d240a1c2c3aa2f7303e7ca4d9721a29f4
diff --git a/vp10/vp10cx.mk b/vp10/vp10cx.mk
index 8d35a9b..3ee97bd 100644
--- a/vp10/vp10cx.mk
+++ b/vp10/vp10cx.mk
@@ -72,6 +72,7 @@
 VP10_CX_SRCS-yes += encoder/resize.c
 VP10_CX_SRCS-yes += encoder/resize.h
 VP10_CX_SRCS-$(CONFIG_INTERNAL_STATS) += encoder/blockiness.c
+VP10_CX_SRCS-$(CONFIG_ANS) += encoder/buf_ans.h
 
 VP10_CX_SRCS-yes += encoder/tokenize.c
 VP10_CX_SRCS-yes += encoder/treewriter.c