Codechange: Implement tile proc handler to test for building bridge over tile.

src/void_cmd.cpp

@@ -87,4 +87,5 @@ extern const TileTypeProcs _tile_type_void_procs = {
 	nullptr,                     // vehicle_enter_tile_proc
 	GetFoundation_Void,       // get_foundation_proc
 	TerraformTile_Void,       // terraform_tile_proc
+	nullptr, // check_build_above_proc
 };