libyang NEW support for user type plugins

Refs #448
diff --git a/Doxyfile.in b/Doxyfile.in
index 682f6e0..475b073 100644
--- a/Doxyfile.in
+++ b/Doxyfile.in
@@ -785,6 +785,7 @@
                          ./src/tree_data.h \
                          ./src/tree_schema.h \
                          ./src/extensions.h \
+                         ./src/user_types.h \
                          ./src/xml.h \
                          ./src/dict.h