Make sure the libpeas-gtk dependency isn't optimized away

Aggressive linkers could drop the dependency which would
cause problems using during runtime.
Based on a patch for the same problem for gedit by Garret Regier.

https://bugzilla.gnome.org/show_bug.cgi?id=739618

origin commit:
https://gitlab.gnome.org/GNOME/eog/commit/46c38b7

src/eom-preferences-dialog.c

@@ -145,6 +145,10 @@ eom_preferences_dialog_class_init (EomPreferencesDialogClass *klass)
 {
 	GtkWidgetClass *widget_class = (GtkWidgetClass*) klass;
 
+	/* This should make sure the libpeas-gtk dependency isn't
+	 * dropped by aggressive linkers (#739618) */
+	g_type_ensure (PEAS_GTK_TYPE_PLUGIN_MANAGER);
+
 	gtk_widget_class_set_template_from_resource (widget_class,
 	                                             "/org/mate/eom/ui/eom-preferences-dialog.ui");
 	gtk_widget_class_bind_template_child_private (widget_class,