Fix GCC 7 warning in the RTS
Fix GCC 7 warning in the RTS

Test Plan: validate

Reviewers: austin, bgamari, erikd, simonmar

Reviewed By: bgamari, simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3648

ezyang/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc