OvmfPkg: Add FileExplorerLib.inf to the dsc file

OvmfPkg: Add FileExplorerLib.inf to the dsc file

Because SecureBootConfigDxe use FileExplorerLib now, but
FileExplorerLib is not in the dsc file of the package
that use SecureBootConfigDxe. Now add it to pass build.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.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: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>