MailSettings: Use the app-mail icon for the window

No app-mail-settings icon exists (yet). app-mail is also what's being
embedded in the executable, so let's stick with it.

Userland/Applications/MailSettings/main.cpp

@@ -23,7 +23,7 @@ int main(int argc, char** argv)
         return 1;
     }
 
-    auto app_icon = GUI::Icon::default_icon("app-mail-settings");
+    auto app_icon = GUI::Icon::default_icon("app-mail");
 
     auto window = MailSettingsWindow::construct();
     window->set_title("Mail Settings");