Add import sharedlist when hasThreadSupport

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).

lib/system/gc_ms.nim

@@ -29,6 +29,9 @@ template mulThreshold(x): expr {.immediate.} = x * 2
 when defined(memProfiler):
   proc nimProfile(requestedSize: int)
   
+when hasThreadSupport:
+  import sharedlist
+
 type
   WalkOp = enum
     waMarkGlobal,  # we need to mark conservatively for global marker procs