Pcbnew: Fix ctrl-MMB double click to perform 'Zoom to Objects'

CHANGED: Ctrl-middle mouse button double click should perform
'Zoom to Objects' in Pcbnew, but a change in modifier key handling
broke it.  This MR restores the functionality.

Fixes https://gitlab.com/kicad/code/kicad/issues/6740

pcbnew/tools/pcb_selection_tool.cpp

@@ -329,7 +329,7 @@ int PCB_SELECTION_TOOL::Main( const TOOL_EVENT& aEvent )
         else if( evt->IsDblClick( BUT_MIDDLE ) )
         {
             // Middle double click?  Do zoom to fit or zoom to objects
-            if( m_exclusive_or ) // Is CTRL key down?
+            if( evt->Modifier( MD_CTRL ) ) // Is CTRL key down?
                 m_toolMgr->RunAction( ACTIONS::zoomFitObjects, true );
             else
                 m_toolMgr->RunAction( ACTIONS::zoomFitScreen, true );