Utils: Fix result of fileNameWithPathComponents()

Combination of bf8bf1adc0 and d83f9d71cc broke
the resulting string.
Fixes tst_fileutils file name tests.

Change-Id: I69c485e52bbf76093b903b407ab2ff2f58d28a0d
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/libs/utils/filepath.cpp

@@ -272,7 +272,7 @@ QString FilePath::fileNameWithPathComponents(int pathComponents) const
         return fullPath.mid(component);
 
     // If there are no more slashes before the found one, return the entire string
-    return displayName();
+    return toString();
 }
 
 /// \returns the base name of the file without the path.