Fix gcc warning (#596)

# Description
Trivial fix of a gcc warning

## Type of change
- [x] Bug fix
- [ ] New feature
- [ ] Documentation update
- [ ] Test update

## Checklist for the reviewer
This checklist should be used as a help for the reviewer.

- [ ] Is the change limited to one issue?
- [ ] Does this PR close the issue?
- [ ] Is the code easy to read and understand?
- [ ] Do all new feature have an accompanying new test?
- [ ] Has the documentation been updated as necessary?

.github/pull_request_template.md

@@ -2,7 +2,7 @@
 
 
 ## Type of change
-- [ ] Bug fix
+- [ ] Bug fix & code cleanup
 - [ ] New feature
 - [ ] Documentation update
 - [ ] Test update

src/pyembed/dlite-python-singletons.c

@@ -208,7 +208,7 @@ PyObject *dlite_python_module_error(DLiteErrors code)
 
   /* Create exception */
   count = snprintf(excname, sizeof(excname), "dlite.%s", errname);
-  assert(count > 0 && count < sizeof(excname));
+  assert(count > 0 && count < (int)sizeof(excname));
   errdescr = dlite_errdescr(code);
 
   if (!(exc = PyErr_NewExceptionWithDoc(excname, errdescr, base, NULL)))