Fix the error caused by last change.

git-svn-id: svn://bind10.isc.org/svn/bind10/branches/trac449@4143 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/cache/rrset_entry.h

@@ -27,6 +27,8 @@ using namespace isc::nsas;
 namespace isc {
 namespace cache {
 
+class RRset;
+
 /// \enum RRset Trustworthiness
 /// For detail of rrset trustworthiness, please refer to
 /// RFC2181 section5.4.1.