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) <[e-mail]>
Reviewed-by: Stefano Stabellini <[e-mail]>
Signed-off-by: Juergen Gross <[e-mail]>

arch/arm/xen/mm.c

···
 	return;
 }
 
-int __init xen_mm_init(void)
+static int __init xen_mm_init(void)
 {
 	struct gnttab_cache_flush cflush;
 	if (!xen_initial_domain())