Fixed compile under c++20 and clang 12

include/spdlog/details/fmt_helper.h

@@ -55,7 +55,7 @@ inline void pad2(int n, memory_buf_t &dest)
     }
     else // unlikely, but just in case, let fmt deal with it
     {
-        fmt::format_to(std::back_inserter(dest), "{:02}", n);
+        fmt::format_to(std::back_inserter(dest), SPDLOG_FMT_RUNTIME("{:02}"), n);
     }
 }