Added const qualifier to stopwatch formatter to support c++20

include/spdlog/stopwatch.h

@@ -61,7 +61,7 @@ template<>
 struct formatter<spdlog::stopwatch> : formatter<double>
 {
     template<typename FormatContext>
-    auto format(const spdlog::stopwatch &sw, FormatContext &ctx) -> decltype(ctx.out())
+    auto format(const spdlog::stopwatch &sw, FormatContext &ctx) const -> decltype(ctx.out())
     {
         return formatter<double>::format(sw.elapsed().count(), ctx);
     }