Hide wxGenericDataViewCtrl helpers in anonymous namespace

This is a good idea anyhow, as these classes don't need to have extern
linkage and so shouldn't have it, and it also fixes some gcc 11
-Wsubobject-linkage warnings.

src/generic/datavgen.cpp

@@ -3816,6 +3816,9 @@ int wxDataViewMainWindow::QueryAndCacheLineHeight(unsigned int row, wxDataViewIt
 }
 
 
+namespace
+{
+
 class RowToTreeNodeJob: public DoJob
 {
 public:
@@ -3868,6 +3871,8 @@ private:
     wxDataViewTreeNode* m_ret;
 };
 
+} // anonymous namespace
+  //
 wxDataViewTreeNode * wxDataViewMainWindow::GetTreeNodeByRow(unsigned int row) const
 {
     wxASSERT( !IsVirtualList() );
@@ -4260,6 +4265,9 @@ int wxDataViewMainWindow::RecalculateCount() const
     }
 }
 
+namespace
+{
+
 class ItemToRowJob : public DoJob
 {
 public:
@@ -4309,6 +4317,8 @@ private:
 
 };
 
+} // anonymous namespace
+
 int
 wxDataViewMainWindow::GetRowByItem(const wxDataViewItem & item,
                                    WalkFlags flags) const