Fix Windows build

rts/win32/OSThreads.c

@@ -226,7 +226,7 @@ forkOS_createThreadWrapper ( void * entry )
 {
     Capability *cap;
     cap = rts_lock();
-    cap = rts_evalStableIO(cap, (HsStablePtr) entry, NULL);
+    rts_evalStableIO(&cap, (HsStablePtr) entry, NULL);
     rts_unlock(cap);
     return 0;
 }