Ensure channel_spacing variable is defined
Ensure channel_spacing variable is defined

Signed-off-by: Colin Holehouse <0a6dd397e1cee93363c8b1218962a1bf096baf83@arm.com>

The-OpenROAD-Project/OpenROAD,The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD,QuantamHD/OpenROAD,QuantamHD/OpenROAD,QuantamHD/OpenROAD,The-OpenROAD-Project/OpenROAD