Enable Sign in as Different User Option in SharePoint 2013

1) Locate and then open the following file in a text editor:
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ TEMPLATE\CONTROLTEMPLATES\Welcome.ascx

2) Add the following element before the existing "ID_RequestAccess" element:

<SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser" Text="<%$Resources:wss,personalactions_loginasdifferentuser%>" Description="<%$Resources:wss,personalactions_loginasdifferentuserdescription%>" MenuGroupId="100" Sequence="100" UseShortId="true" />

3) Save the file