Add FileExplorerLib to SecurePkg DSC to pass build, as SecureBootConfigDxe requires this library now. Cc: Chao Zhang <chao.b.zhang@intel.com> Cc: Eric Dong <eric.dong@intel.com> Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Dandan Bi <dandan.bi@intel.com> Reviewed-by: Eric Dong <eric.dong@intel.com> Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>