Add missing comma

Found by new clang-tidy checker
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118534113

Author: bergeret

src/googleclouddebugger/immutability_tracer.cc

@@ -62,7 +62,7 @@ static const char* kWhitelistedCFunctions[] = {
   "list",
   "unichr",
   "format",
-  "locals"
+  "locals",
   "reduce",
   "unicode",
   "chr",