GraphML witness: creationtime is a graph attribute

SV-COMP requires that this is set, which we take care of in your sv-comp
wrapper script.

Author: tautschnig

src/xmllang/graphml.cpp

@@ -402,6 +402,16 @@ bool write_graphml(const graphmlt &src, std::ostream &os)
     key.set_attribute("id", "producer");
   }
 
+  // <key attr.name="creationtime" attr.type="string" for="graph"
+  //      id="creationtime"/>
+  {
+    xmlt &key = graphml.new_element("key");
+    key.set_attribute("attr.name", "creationtime");
+    key.set_attribute("attr.type", "string");
+    key.set_attribute("for", "graph");
+    key.set_attribute("id", "creationtime");
+  }
+
   // <key attr.name="startline" attr.type="int" for="edge" id="startline"/>
   {
     xmlt &key=graphml.new_element("key");