symlink_outputs: Add .KATI_SYMLINK_OUTPUTS for art_apex_symlink_timestamp

Test: m
Bug: 160568334
Change-Id: Ic2de276e52a1ab84140fed7e1eed91e463f6b267
diff --git a/Android.mk b/Android.mk
index 3a21929..5b829f9 100644
--- a/Android.mk
+++ b/Android.mk
@@ -362,6 +362,7 @@
 	$(hide) mkdir -p $(dir $(PRIVATE_LINK_NAME))
 	$(hide) ln -sf $(TARGET_ART_APEX) $(PRIVATE_LINK_NAME)
 	$(hide) touch $@
+$(art_apex_symlink_timestamp): .KATI_SYMLINK_OUTPUTS := $(PRIVATE_LINK_NAME)
 
 art_apex_manifest_file :=