linter: Fix missing comma

Reported by Semmle's LGTM
(https://lgtm.com/projects/g/diffblue/cbmc/alerts/?mode=list).

Author: tautschnig

scripts/cpplint.py

@@ -222,7 +222,7 @@
     'readability/constructors',
     'readability/dowhile',
     'readability/fn_size',
-    'readability/function_comment'
+    'readability/function_comment',
     'readability/identifier_spacing',
     'readability/identifiers',
     'readability/inheritance',