sm6225-common: Import `hardware/xiaomi` namespace

Change-Id: I9aca52286bcf8dd2efad42245dfff36cda02c24e
diff --git a/common.mk b/common.mk
index 15c5234..5caf0e1 100644
--- a/common.mk
+++ b/common.mk
@@ -65,7 +65,8 @@
 
 # Soong namespaces
 PRODUCT_SOONG_NAMESPACES += \
-    $(LOCAL_PATH)
+    $(LOCAL_PATH) \
+    hardware/xiaomi
 
 # Update engine
 PRODUCT_PACKAGES += \