VTS: Add flag dependency so librenderengine can be used

I6c436b6b80b27930d82aabf22107216172773b56 adds a dependency for
librenderengine on flags. Add that dependency so we can use
librenderengine here, too.

Bug: 305445199

Test: atest VtsHalGraphicsComposerV2_2TargetTest
Test: atest VtsHalGraphicsComposer3_TargetTest
Change-Id: If339ac94a6eff4af34e60cf1c1135289b8915441