xen: mm: make xen_mm_init static

The xen_mm_init is not exported or used outside of the file
it is declared in, so make it static. This fixes the following
sparse warning:

arch/arm/xen/mm.c:136:12: warning: symbol 'xen_mm_init' was not declared. Should it be static?

Signed-off-by: Ben Dooks (Codethink) <ben.dooks@codethink.co.uk>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>