ShellPkg: Fix compiler warning 'unused variable'

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Harry Liebel <Harry.Liebel@arm.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>


git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/ShellPkg@15462 6f19259b-4bc3-4df7-8a09-765794883524