Change the visibility of the Slf4jLogger constructor (#1994)

Co-authored-by: Marvin Froeder <velo@users.noreply.github.com>

slf4j/src/main/java/feign/slf4j/Slf4jLogger.java

@@ -40,7 +40,7 @@ public class Slf4jLogger extends feign.Logger {
     this(LoggerFactory.getLogger(name));
   }
 
-  Slf4jLogger(Logger logger) {
+  public Slf4jLogger(Logger logger) {
     this.logger = logger;
   }