Use LinkerInternals.h for exitLinker.

Part of remove HEAP_ALLOCED patch set (#8199)

Summary: Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonmar, austin

Subscribers: simonmar, ezyang, carter, thomie

Differential Revision: https://phabricator.haskell.org/D262

GHC Trac Issues: #8199

rts/RtsStartup.c

@@ -36,7 +36,7 @@
 #include "Timer.h"
 #include "Globals.h"
 #include "FileLock.h"
-void exitLinker( void );        // there is no Linker.h file to include
+#include "LinkerInternals.h"
 
 #if defined(PROFILING)
 # include "ProfHeap.h"