Oilpan: fix Windows build after r178431.

Source/platform/heap/Visitor.h

Index: Source/platform/heap/Visitor.h
diff --git a/Source/platform/heap/Visitor.h b/Source/platform/heap/Visitor.h
index f9dffd5ba41ed4aa4411fef73c7e58141b59cdc6..4bb1b4aeaccc08251463b3f3e159fefe710bbbcf 100644
--- a/Source/platform/heap/Visitor.h
+++ b/Source/platform/heap/Visitor.h
@@ -584,7 +584,7 @@ template<typename T> bool ObjectAliveTrait<T>::isAlive(Visitor* visitor, T* obj)
 // Note that this is only enabled for Member<B>. For Member<A> which we can
 // compute the object header addr statically, this dynamic dispatch is not used.
 
-class GarbageCollectedMixin {
+class PLATFORM_EXPORT GarbageCollectedMixin {
 public:
     virtual void adjustAndMark(Visitor*) const { };
     virtual bool isAlive(Visitor*) const { return true; };