XER support

diff --git a/skeletons/constraints.c b/skeletons/constraints.c
index 862d8f1..d95e3cb 100644
--- a/skeletons/constraints.c
+++ b/skeletons/constraints.c
@@ -1,3 +1,4 @@
+#include <asn_internal.h>
 #include <constraints.h>
 #include <constr_TYPE.h>