month-view: match style context state from child

We were mistakenly using the style context of the view
to retrieve the child's margin.

Fix that by retrieving the child margin from the child's
style context.

src/gcal-month-view.c

@@ -1293,9 +1293,11 @@ gcal_month_view_size_allocate (GtkWidget     *widget,
 
           if (size_left[i] > natural_height)
             {
-              gtk_style_context_get_margin (gtk_widget_get_style_context (child_widget),
-                                            gtk_style_context_get_state (context),
-                                            &margin);
+              GtkStyleContext *child_context;
+
+              child_context = gtk_widget_get_style_context (child_widget);
+
+              gtk_style_context_get_margin (child_context, gtk_style_context_get_state (child_context), &margin);
 
               pos_x = cell_width * (i % 7) + margin.left;
               pos_y = cell_height * ((i / 7) + first_row_gap) + start_grid_y + margin.top;