[gnome-maps/wip/mlundblad/cleanup-gtk-deprecations] mainWindow: Stop using generic GtkContainer add method

[Marcus Lundblad <mlundblad src gnome org>]

commit cde191e2e1d5d2e0c0f3b9f4f12862df3aa2d97c
Author: Marcus Lundblad <ml update uu se>
Date:   Wed Mar 24 22:01:26 2021 +0100

    mainWindow: Stop using generic GtkContainer add method
    
    In preparation for GTK 4 use class-specific methods
    to add and remove children when possible.

 src/mainWindow.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/mainWindow.js b/src/mainWindow.js
index 47036c8f..5d75f992 100644
--- a/src/mainWindow.js
+++ b/src/mainWindow.js
@@ -103,7 +103,7 @@ var MainWindow = GObject.registerClass({
                 MapView.MapType.LOCAL : undefined,
             mainWindow: this });
 
-        this._mainGrid.add(this._mapView);
+        this._mainGrid.attach(this._mapView, 0, 0, 1, 1);
 
         this._mapView.gotoUserLocation(false);