Really mark sdk_addon as PHONY to fix `m snod`

When we're using a dont_bother_goals (including snod), we don't load
build/make/core/tasks/*.mk, so sdk_addon isn't defined as a PHONY rule.

This causes a build break because build/make/core/Makefile uses
dist-for-goals with sdk_addon. dist-for-goals requires its arguments to
be PHONY targets.

Test: m snod
Change-Id: I69937fe9360be57766d17ed4c28b5d2a1e348308
diff --git a/core/main.mk b/core/main.mk
index 461c3b4..6374be9 100644
--- a/core/main.mk
+++ b/core/main.mk
@@ -1415,7 +1415,7 @@
 .PHONY: docs
 docs: $(ALL_DOCS)
 
-.PHONY: sdk win_sdk winsdk-tools
+.PHONY: sdk win_sdk winsdk-tools sdk_addon
 ALL_SDK_TARGETS := $(INTERNAL_SDK_TARGET)
 sdk: $(ALL_SDK_TARGETS)
 $(call dist-for-goals,sdk win_sdk, \