gtk: Don't use deprecated gtk_image_menu_item_new_with_mnemonic

gtk: Don't use deprecated gtk_image_menu_item_new_with_mnemonic

In these cases we weren't using an image in the menu item anyways, so
just do as the suggestion says. Should be fine for all qemu supported
gtk versions.

ui/gtk.c: In function ‘gd_create_menu_machine’:
ui/gtk.c:1284:5: error: ‘gtk_image_menu_item_new_with_mnemonic’ is deprecated (declared at /usr/include/gtk-3.0/gtk/deprecated/gtkimagemenuitem.h:76): Use 'gtk_menu_item_new_with_mnemonic' instead [-Werror=deprecated-declarations]
     s->reset_item = gtk_image_menu_item_new_with_mnemonic(_("_Reset"));
     ^
ui/gtk.c:1287:5: error: ‘gtk_image_menu_item_new_with_mnemonic’ is deprecated (declared at /usr/include/gtk-3.0/gtk/deprecated/gtkimagemenuitem.h:76): Use 'gtk_menu_item_new_with_mnemonic' instead [-Werror=deprecated-declarations]
     s->powerdown_item = gtk_image_menu_item_new_with_mnemonic(_("Power _Down"));

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>