Fix bug with dragging non-draggable columns in wxMSW wxHeaderCtrl.

Properly ignore HDN_BEGINDRAG events for the columns without wxCOL_REORDERABLE
flag. This fixes dragging non-draggable columns in wxDataViewCtrl under MSW.

Closes #14940.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73389 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/headerctrl.h

@@ -127,6 +127,9 @@ private:
     // the offset of the window used to emulate scrolling it
     int m_scrollOffset;
 
+    // actual column we are dragging or -1 if not dragging anything
+    int m_colBeingDragged;
+
     wxDECLARE_NO_COPY_CLASS(wxHeaderCtrl);
 };