skins2: fix forgotten initialization in copy constructor

This bug was unnoticed because this constructor is only called in a situation
where really copying from origin or leaving it to the compiler to set it via
the default constructor leads to the same result (an empty list). Yet, this
was a bug.

It may solve trac #6599 though,
(a Linux port where there is a real initialization issue)

modules/gui/skins2/utils/var_tree.cpp

@@ -58,7 +58,8 @@ VarTree::VarTree( intf_thread_t *pIntf, VarTree *pParent, int id,
 }
 
 VarTree::VarTree( const VarTree& v )
-    : Variable( v.getIntf() ), m_pParent( v.m_pParent ),
+    : Variable( v.getIntf() ),
+      m_children( v.m_children), m_pParent( v.m_pParent ),
       m_id( v.m_id ), m_cString( v.m_cString ),
       m_readonly( v.m_readonly ), m_selected( v.m_selected ),
       m_playing( v.m_playing ), m_expanded( v.m_expanded ),