ShellPkg: Fixed build error 'variable set but not used'

GCC toolchain cannot build ShellPkg.

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

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14326 6f19259b-4bc3-4df7-8a09-765794883524