st-hal-ar: Guard on oss sthal flag

Most devices use prebuilt soundtrigger and this should not conflict

Change-Id: Ib57170f7cc826a2739eb3428d9839ce754c27319
diff --git a/Android.mk b/Android.mk
index 07904cb..d50ce71 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1,4 +1,5 @@
 ifneq ($(AUDIO_USE_STUB_HAL), true)
+ifeq ($(strip $(BOARD_SUPPORTS_OPENSOURCE_STHAL)),true)
 ifeq ($(TARGET_USES_QCOM_MM_AUDIO),true)
 ifeq ($(TARGET_USES_QCOM_AUDIO_AR),true)
 
@@ -58,4 +59,5 @@
 include $(BUILD_SHARED_LIBRARY)
 endif #TARGET_USES_QCOM_AUDIO_AR
 endif #TARGET_USES_QCOM_MM_AUDIO
+endif #BOARD_SUPPORTS_OPENSOURCE_STHAL
 endif #AUDIO_USE_STUB_HAL