PNS: Don't link unroutable items

The joint map only works for items that are routable, so don't bother
adding unroutable items as this only takes additional time to process

Fixes #3976 | https://gitlab.com/kicad/code/kicad/issues/3976

pcbnew/router/pns_node.cpp

@@ -538,7 +538,9 @@ const ITEM_SET NODE::HitTest( const VECTOR2I& aPoint ) const
 
 void NODE::addSolid( SOLID* aSolid )
 {
+    if( aSolid->IsRoutable() )
         linkJoint( aSolid->Pos(), aSolid->Layers(), aSolid->Net(), aSolid );
+
     m_index->Add( aSolid );
 }