Pcbnew, Draw zones in GAL mode: fix missing initialization of line color.

It is noticeable only in printing mode.

Fixes: lp:1818971
https://bugs.launchpad.net/kicad/+bug/1818971

pcbnew/pcb_painter.cpp

@@ -1140,6 +1140,7 @@ void PCB_PAINTER::draw( const ZONE_CONTAINER* aZone, int aLayer )
             return;
 
         // Set up drawing options
+        m_gal->SetStrokeColor( color );
         m_gal->SetFillColor( color );
         m_gal->SetLineWidth( aZone->GetMinThickness() );