Add default way for switch in function 'RunCommandOrFile' of 'Shell.c'.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Shumin Qiu <shumin.qiu@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15018 6f19259b-4bc3-4df7-8a09-765794883524