enable wextra in vp9

Change-Id: Ibb8bd0c256d138a21e8f96feed39f0793ed98d20

libs.mk

diff --git a/libs.mk b/libs.mk
index 25fbc2c..c7c2748 100644
--- a/libs.mk
+++ b/libs.mk
@@ -133,6 +133,8 @@
   CODEC_DOC_SECTIONS += vp9 vp9_decoder
 endif
 
+VP9_PREFIX=vp9/
+$(BUILD_PFX)$(VP9_PREFIX)%.c.o: CFLAGS += -Wextra
 
 ifeq ($(CONFIG_ENCODERS),yes)
   CODEC_DOC_SECTIONS += encoder