hal: Append the local header libraries

As part of the VNDK changes, libhardware_headers is being
added as local header libraries before audio_kernel_headers.
So, append the local header libraries for audio_kernel_headers
to avoid overwrite.

Change-Id: Ie184930eaaa9fd425f73fc13a2fdc849f3c01e94
diff --git a/hal/Android.mk b/hal/Android.mk
index 4534554..4d107e5 100644
--- a/hal/Android.mk
+++ b/hal/Android.mk
@@ -80,7 +80,7 @@
 LOCAL_ADDITIONAL_DEPENDENCIES += $(TARGET_OUT_INTERMEDIATES)/KERNEL_OBJ/usr
 
 ifeq ($(strip $(AUDIO_FEATURE_ENABLED_DLKM)),true)
-  LOCAL_HEADER_LIBRARIES := audio_kernel_headers
+  LOCAL_HEADER_LIBRARIES += audio_kernel_headers
   LOCAL_C_INCLUDES += $(TARGET_OUT_INTERMEDIATES)/vendor/qcom/opensource/audio-kernel/include
   LOCAL_ADDITIONAL_DEPENDENCIES += $(BOARD_VENDOR_KERNEL_MODULES)
 endif
diff --git a/post_proc/Android.mk b/post_proc/Android.mk
index dbc068c..42763ed 100644
--- a/post_proc/Android.mk
+++ b/post_proc/Android.mk
@@ -70,7 +70,7 @@
         $(call include-path-for, audio-effects)
 
 ifeq ($(strip $(AUDIO_FEATURE_ENABLED_DLKM)),true)
-  LOCAL_HEADER_LIBRARIES := audio_kernel_headers
+  LOCAL_HEADER_LIBRARIES += audio_kernel_headers
   LOCAL_C_INCLUDES += $(TARGET_OUT_INTERMEDIATES)/vendor/qcom/opensource/audio-kernel/include
   LOCAL_ADDITIONAL_DEPENDENCIES += $(BOARD_VENDOR_KERNEL_MODULES)
 endif
@@ -180,7 +180,7 @@
         external/tinycompress/include
 
 ifeq ($(strip $(AUDIO_FEATURE_ENABLED_DLKM)),true)
-  LOCAL_HEADER_LIBRARIES := audio_kernel_headers
+  LOCAL_HEADER_LIBRARIES += audio_kernel_headers
   LOCAL_C_INCLUDES += $(TARGET_OUT_INTERMEDIATES)/vendor/qcom/opensource/audio-kernel/include
   LOCAL_ADDITIONAL_DEPENDENCIES += $(BOARD_VENDOR_KERNEL_MODULES)
 endif