audio: update references to support prebuilt kernel build

Updating kernel references to LOCAL_ADDITIONAL_DEPENDENCIES and
BOARD_KERNEL_HEADER_DIR

Change-Id: I94fedd691dba7878b25c0ddbca0c87e62cbc856d
diff --git a/msm8909/audiod/Android.mk b/msm8909/audiod/Android.mk
index 9105787..d45ec1e 100644
--- a/msm8909/audiod/Android.mk
+++ b/msm8909/audiod/Android.mk
@@ -14,6 +14,8 @@
 	libbinder \
 	libmedia
 
+LOCAL_ADDITIONAL_DEPENDENCIES := $(BOARD_KERNEL_HEADER_DEPENDENCIES)
+
 LOCAL_MODULE:= audiod
 
 include $(BUILD_EXECUTABLE)