[subtree] add xlnt to subtree config file

Author: amin1377

dev/subtree_config.xml

@@ -54,4 +54,9 @@
         internal_path="parmys"
         external_url="https://github.com/CAS-Atlantic/parmys-plugin.git"
         default_external_ref="v3.0"/>
+    <subtree
+        name="xlnt"
+        internal_path="libs/EXTERNAL/xlnt"
+        external_url="https://github.com/tfussell/xlnt.git"
+        default_external_ref="master"/>
 </subtrees>