add a user-provided constructor to emulated c++11 NullPtr

Source/wtf/NullPtr.h

Index: Source/wtf/NullPtr.h
diff --git a/Source/wtf/NullPtr.h b/Source/wtf/NullPtr.h
index b6b34e444160a8fb0054ef30f705a6ab7d7d534d..83c9687306f8ae44362d12b9203a1cd5f76ae6c4 100644
--- a/Source/wtf/NullPtr.h
+++ b/Source/wtf/NullPtr.h
@@ -49,6 +49,12 @@ typedef decltype(nullptr) nullptr_t;
 namespace std {
 class nullptr_t {
 public:
+    // Required in order to create const nullptr_t objects without an
+    // explicit initializer in GCC 4.5, a la:
+    //
+    // const std::nullptr_t nullptr;
+    nullptr_t() { }
+
     // Make nullptr convertible to any pointer type.
     template<typename T> operator T*() const { return 0; }
     // Make nullptr convertible to any member pointer type.