router: Take track width in dp from dp

Fixes and issue where forces were calculated using the current track
width even in differential placer mode where the tracks are specified by
the differential pair width.

Fixes: lp:1814480
* https://bugs.launchpad.net/kicad/+bug/1814480

pcbnew/router/pns_diff_pair_placer.cpp

@@ -130,7 +130,7 @@ bool DIFF_PAIR_PLACER::propagateDpHeadForces ( const VECTOR2I& aP, VECTOR2I& aNe
     else
     {
         virtHead.SetLayer( m_currentLayer );
-        virtHead.SetDiameter( m_sizes.DiffPairGap() + 2 * m_sizes.TrackWidth() );
+        virtHead.SetDiameter( m_sizes.DiffPairGap() + 2 * m_sizes.DiffPairWidth() );
     }
 
     VECTOR2I lead( 0, 0 );// = aP - m_currentStart ;