ShellPkg: Add a typecast to 'BOOLEAN' to avoid build failure in VS2005. 

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qiu Shumin <shumin.qiu@intel.com>
Reviewed-by: Jaben Carsey <Jaben.carsey@intel.com>


git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16199 6f19259b-4bc3-4df7-8a09-765794883524