[Qt] Fix the JSC build on Mac

Unreviewed, build fix.

* heap/HeapTimer.h:
Qt on Mac has USE(CF) true, and should use the CF HeapTimer in that case.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@141097 268f45cc-cd09-0410-ab3c-d52691b4dbfc