auth: Fix a 'temporary used in loop' warning reported by g++ 11.1.0

auth: Fix a 'temporary used in loop' warning reported by g++ 11.1.0

```
common_startup.cc: In function ‘void mainthread()’:
common_startup.cc:617:24: warning: loop variable ‘algotype’ of type ‘const string&’ {aka ‘const std::__cxx11::basic_string<char>&’} binds to a temporary constructed from type ‘const char* const’ [-Wrange-loop-construct]
  617 |     for (const string& algotype : {"ksk", "zsk"}) {
      |                        ^~~~~~~~
common_startup.cc:617:24: note: use non-reference type ‘const string’ {aka ‘const std::__cxx11::basic_string<char>’} to make the copy explicit or ‘const char* const&’ to prevent copying
```