Actually reference the fixed Proguard flags.

Bug: 2869888
Change-Id: I3277b51cd411c7933607c08b6022870ef83e9197
diff --git a/packages/SystemUI/Android.mk b/packages/SystemUI/Android.mk
index 4c83768..910e84e 100644
--- a/packages/SystemUI/Android.mk
+++ b/packages/SystemUI/Android.mk
@@ -10,4 +10,6 @@
 LOCAL_PACKAGE_NAME := SystemUI
 LOCAL_CERTIFICATE := platform
 
+LOCAL_PROGUARD_FLAGS := -include $(LOCAL_PATH)/proguard.flags
+
 include $(BUILD_PACKAGE)