Constify callback structures

None of the callback structures is modified at runtime, so mark them as
const. This allows them to go into the read-only section.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
diff --git a/xml.c b/xml.c
index 09dabc7..5e42cc4 100644
--- a/xml.c
+++ b/xml.c
@@ -279,7 +279,7 @@
 	return xml_create_context_mem(ctx->xml, strlen(ctx->xml));
 }
 
-static struct iio_backend_ops xml_ops = {
+static const struct iio_backend_ops xml_ops = {
 	.clone = xml_clone,
 };