- remove also constant derivatives from known vars as the other self generated vars

git-svn-id: https://openmodelica.org/svn/OpenModelica/trunk@13520 f25d12d1-65f4-0310-ae8a-bbce733d8d8e

Author: Jens Frenkel

Compiler/BackEnd/BackendVariable.mo

@@ -4050,6 +4050,7 @@ algorithm
     case DAE.CREF_QUAL(ident = "$ZERO") then true;
     case DAE.CREF_QUAL(ident = "$_DER") then true;
     case DAE.CREF_QUAL(ident = "$pDER") then true;
+    case DAE.CREF_QUAL(ident = "$DER") then true;
     else then false;
   end match;
 end selfGeneratedVar;