Fix forward declaration of RefHolderBase in wrong namespace.

RefHolderBase is in the namespace edm::reftobase, but it was
forward declared in namespace edm. This could cause a compilation
error when instantiating this template under some circumstances.

This patch moves the forward declaration to the correct namespace.

DataFormats/Common/interface/BaseHolder.h

@@ -9,9 +9,9 @@
 
 namespace edm {
   class ProductID;
-  class RefHolderBase;
 
   namespace reftobase {
+    class RefHolderBase;
     template<typename T> class BaseVectorHolder;
     class RefVectorHolderBase;