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/post_proc/Android.mk b/msm8909/post_proc/Android.mk
index 50f33e9..5de115c 100644
--- a/msm8909/post_proc/Android.mk
+++ b/msm8909/post_proc/Android.mk
@@ -30,6 +30,7 @@
 
 LOCAL_C_INCLUDES := \
 	external/tinyalsa/include \
+	$(BOARD_KERNEL_HEADER_DIR) \
 	$(call include-path-for, audio-effects)
 
 include $(BUILD_SHARED_LIBRARY)