rts/RetainerProfile: Const-correctness fixes
rts/RetainerProfile: Const-correctness fixes

These were found while using Hadrian, which apparently uses slightly
stricter warning flags than the make-based build system.

Test Plan: Validate

Reviewers: austin, erikd, simonmar

Reviewed By: erikd

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3679

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