audio hal: explicitly import headers

Test: works with BOARD_VNDK_VERSION := current
Bug: 33241851
Change-Id: I34983a5527a16e8e9f5a6a87edac0b741aeb67d6
diff --git a/audio/2.0/default/Android.mk b/audio/2.0/default/Android.mk
index 93f7ad0..621853c 100644
--- a/audio/2.0/default/Android.mk
+++ b/audio/2.0/default/Android.mk
@@ -43,6 +43,12 @@
     android.hardware.audio.common@2.0 \
     android.hardware.audio.common@2.0-util \
 
+LOCAL_HEADER_LIBRARIES := \
+    libaudioclient_headers \
+    libaudio_system_headers \
+    libhardware_headers \
+    libmedia_headers \
+
 LOCAL_WHOLE_STATIC_LIBRARIES := libmedia_helper
 
 include $(BUILD_SHARED_LIBRARY)