Xen/swiotlb: mark xen_swiotlb_fixup() __init

It's sole user (pci_xen_swiotlb_init()) is __init, too.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

Message-ID: <e1198286-99ec-41c1-b5ad-e04e285836c9@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>