additional log overload

calling log with a string_view as msg called
```
 template<typename... Args>
    void log(source_loc loc, level::level_enum lvl, string_view_t fmt, const Args &... args)
```

instead of
```
template<class T, typename std::enable_if<std::is_convertible<const T &, spdlog::string_view_t>::value, T>::type * = nullptr>
    void log(source_loc loc, level::level_enum lvl, const T &msg)
```

which lead to an unnecessary call to fmt::format

include/spdlog/logger.h

@@ -143,6 +143,11 @@ public:
     // T can be statically converted to string_view
     template<class T, typename std::enable_if<std::is_convertible<const T &, spdlog::string_view_t>::value, T>::type * = nullptr>
     void log(source_loc loc, level::level_enum lvl, const T &msg)
+    {
+        log(loc,lvl, string_view_t{msg});
+    }
+
+    void log(source_loc loc, level::level_enum lvl, string_view_t msg)
     {
         bool log_enabled = should_log(lvl);
         bool traceback_enabled = tracer_.enabled();