On your facebook Timeline profile, click on the "Friends" box at the top. This will take you to your Friends List page.
On the friends list page, click on the "Edit" button (upper right). You can then customize "Who can see your full friend list on your timeline".
To change it, click on the drop down box and select one of the following:
- Only Me
- Close Friends
Note: mutual friends will still be visible to other people.