xen/arm: Fix compile errors when drivers are compiled as modules.

We end up with:

ERROR: "HYPERVISOR_event_channel_op" [drivers/xen/xen-gntdev.ko] undefined!
ERROR: "privcmd_call" [drivers/xen/xen-privcmd.ko] undefined!
ERROR: "HYPERVISOR_grant_table_op" [drivers/net/xen-netback/xen-netback.ko] undefined!

and this patch exports said function (which is implemented in hypercall.S).
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/arm/xen/enlighten.c

@@ -166,3 +166,8 @@ void free_xenballooned_pages(int nr_pages, struct page **pages)
 	*pages = NULL;
 }
 EXPORT_SYMBOL_GPL(free_xenballooned_pages);
+
+/* In the hypervisor.S file. */
+EXPORT_SYMBOL_GPL(HYPERVISOR_event_channel_op);
+EXPORT_SYMBOL_GPL(HYPERVISOR_grant_table_op);
+EXPORT_SYMBOL_GPL(privcmd_call);