nonmoving: Fix small CPP bug

Previously an incorrect semicolon meant that we would fail to call
busy_wait_nop when spinning.

rts/sm/NonMovingMark.c

@@ -737,9 +737,11 @@ void updateRemembSetPushStack(Capability *cap, StgStack *stack)
             // The concurrent GC has claimed the right to mark the stack.
             // Wait until it finishes marking before proceeding with
             // mutation.
-            while (needs_upd_rem_set_mark((StgClosure *) stack));
+            while (needs_upd_rem_set_mark((StgClosure *) stack))
 #if defined(PARALLEL_GC)
                 busy_wait_nop(); // TODO: Spinning here is unfortunate
+#else
+                ;
 #endif
             return;
         }