268fa5f3bc25 grafted an old patch to define --features=servo in
rules.mk. That patch was written before RUST_LIBRARY_FEATURES
existed. This commit fixes it up.
MozReview-Commit-ID: L5atm5CsP8d
--HG--
extra : amend_source : 9362db15a696ebd5871df94afb429d6f828de184