audio: Fix compilation issue

Add audio-driver headers files path to search
directory list during compilation process.

Change-Id: Ie7c5b50a7bcdfb62af33f83dec64936c6b53cabf
diff --git a/post_proc/Makefile.am b/post_proc/Makefile.am
index e161f8d..54602a2 100644
--- a/post_proc/Makefile.am
+++ b/post_proc/Makefile.am
@@ -1,4 +1,5 @@
 AM_CFLAGS = -I ${WORKSPACE}/external/tinyalsa/include \
+            -I $(PKG_CONFIG_SYSROOT_DIR)/usr/include/audio-kernel \
             -I ${WORKSPACE}/system/media/audio_effects/include
 
 c_sources = bundle.c \