Without this change, a user's Nim code won't compile if they're using both threads & the mark-and-sweep GC:
lib/system/gc_ms.nim(75, 18) Error: undeclared identifier: 'SharedList'
toDispose: SharedList[pointer]
^
This small code block addition was copied from "lib/system/gc.nim" (where it appears directly after a `when defined(memProfiler)` block also).