Remove -Waggregate-return when building RTS

This causes slow validate build to fail (in Profiling.c:countTickss), and
there's nothing wrong with struct returns.

Reviewers: simonmar, bgamari, erikd

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5164

rts/ghc.mk

@@ -329,7 +329,6 @@ WARNING_OPTS += -Wstrict-prototypes
 WARNING_OPTS += -Wmissing-prototypes 
 WARNING_OPTS += -Wmissing-declarations
 WARNING_OPTS += -Winline
-WARNING_OPTS += -Waggregate-return
 WARNING_OPTS += -Wpointer-arith
 WARNING_OPTS += -Wmissing-noreturn
 WARNING_OPTS += -Wnested-externs