Fix proguard flag file dependency.

Change-Id: Icd30226cfaa943648f8724b4208a22c661070262
diff --git a/packages/SystemUI/Android.mk b/packages/SystemUI/Android.mk
index cfad939..2c13069 100644
--- a/packages/SystemUI/Android.mk
+++ b/packages/SystemUI/Android.mk
@@ -13,6 +13,6 @@
 LOCAL_PACKAGE_NAME := SystemUI
 LOCAL_CERTIFICATE := platform
 
-LOCAL_PROGUARD_FLAGS := -include $(LOCAL_PATH)/proguard.flags
+LOCAL_PROGUARD_FLAG_FILES := proguard.flags
 
 include $(BUILD_PACKAGE)