Update xsim import to function with lstlisting

gummi/template/courses.tex

@@ -36,7 +36,7 @@
 \setcounter{secnumdepth}{3}
 
 % Exercises
-\usepackage[clear-aux]{xsim}
+\usepackage[clear-aux,use-files]{xsim}
 \usepackage{substr}
 \usepackage{ifthen}
 \usepackage{environ}