xen/pvcalls: Add MODULE_LICENSE()

Since commit ba1029c9 ("modpost: detect modules without a
MODULE_LICENSE") modules without said macro will generate

WARNING: modpost: missing MODULE_LICENSE() in <filename>

While at it, also add module description and attribution.
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

drivers/xen/pvcalls-back.c

@@ -1238,3 +1238,7 @@ static void __exit pvcalls_back_fin(void)
 }
 
 module_exit(pvcalls_back_fin);
+
+MODULE_DESCRIPTION("Xen PV Calls backend driver");
+MODULE_AUTHOR("Stefano Stabellini <sstabellini@kernel.org>");
+MODULE_LICENSE("GPL");

drivers/xen/pvcalls-front.c

@@ -1273,3 +1273,7 @@ static int __init pvcalls_frontend_init(void)
 }
 
 module_init(pvcalls_frontend_init);
+
+MODULE_DESCRIPTION("Xen PV Calls frontend driver");
+MODULE_AUTHOR("Stefano Stabellini <sstabellini@kernel.org>");
+MODULE_LICENSE("GPL");