tokenlist.h: fixed google-explicit-constructor clang-tidy and `noEx…

…plicitConstructor` selfcheck warning

lib/tokenlist.h

@@ -44,7 +44,7 @@ namespace simplecpp {
  * @brief This struct stores pointers to the front and back tokens of the list this token is in.
  */
 struct TokensFrontBack {
-    TokensFrontBack(const TokenList& list) : list(list) {}
+    explicit TokensFrontBack(const TokenList& list) : list(list) {}
     Token *front{};
     Token* back{};
     const TokenList& list;