eml: fix undefined vars on <Perl 5.28
Encode::MIME::Header::_decode_octets did not correctly default
to Encode::FB_DEFAULT until Encode 2.93 (perl5.git commit
0c541dc5633a341cf44b818014b58e7f8be532e9). Provide the default
again to work with older Perls.
Reported-by: Ali Alnubani <alialnu@nvidia.com>
Link: https://public-inbox.org/meta/DM6PR12MB49106F8E3BD697B63B943A22DADB0@DM6PR12MB4910.namprd12.prod.outlook.com/
Tested-by: Ali Alnubani <alialnu@nvidia.com>
(cherry picked from commit
46bd595f57cc3d425754b0d0770c125616e74448)