SharePoint 2013 - Enable Sign in as a Different User

1. Find and open the following file from the Sharepoint 15 hive
 'C:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\TEMPLATE\CONTROLTEMPLATES\Welcome.ascx'

2. Add the following control code before the control having ID 'id="ID_RequestAccess"' and save the file.
<SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser"
 Text="<%$Resources:wss,personalactions_loginasdifferentuser%>"
 Description="<%$Resources:wss,personalactions_loginasdifferentuserdescription%>"
 MenuGroupId="100"
 Sequence="100"
 UseShortId="true"
 />

3. Reload the SharePoint web application and check again. 'Sign in as a Different User' should be visible now :)