[v3] hashtable fw decl fix

[Benjamin De Kosnik]

This patchlette needed for versioned builds, just makes
declaration/foward declaration consistent.

-benjamin

tested x86/linux
tested x86/linux versioned namespaces
diff --git a/libstdc++-v3/include/bits/hashtable_policy.h b/libstdc++-v3/include/bits/hashtable_policy.h
index 3b43510..a42d3d3 100644
--- a/libstdc++-v3/include/bits/hashtable_policy.h
+++ b/libstdc++-v3/include/bits/hashtable_policy.h
@@ -33,12 +33,16 @@
 
 namespace std _GLIBCXX_VISIBILITY(default)
 {
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+
   template<typename _Key, typename _Value, typename _Alloc,
 	   typename _ExtractKey, typename _Equal,
 	   typename _H1, typename _H2, typename _Hash,
 	   typename _RehashPolicy, typename _Traits>
     class _Hashtable;
 
+_GLIBCXX_END_NAMESPACE_VERSION
+
 namespace __detail
 {
 _GLIBCXX_BEGIN_NAMESPACE_VERSION