Fix performance warning message

Author: andrykonchin

src/main/java/org/truffleruby/language/PerformanceWarningNode.java

@@ -68,7 +68,7 @@ private void log(String message) {
                 .getTopMostUserSourceSection(getEncapsulatingSourceSection());
 
         final String displayedWarning = String.format(
-                "%s: warning: %s\n",
+                "%s: warning: %s%n",
                 getContext().fileLine(userSourceSection),
                 message);