Always define sdk_addon as phony

dist-write-files will add dist targets to the sdk_addon target,
but the sdk_addon target is not declared as phony when building
with mm (ONE_SHOT_MAKEFILE set), causing:
build/make/core/main.mk:1495: error: writing to readonly directory: "sdk_addon"

Always define sdk_addon as phony even if ONE_SHOT_MAKEFILE is set.

Bug: 118144231
Test: mm
Change-Id: I99d14a98a7597ebd694c765f94b0d6f4486860dc