[bug-mailutils] Add pt_BR to source code

[Rafael Fontenelle]

Hello,

In 2018-04-25 I sent first Brazilian Portuguese translation for
mailutils 3.3 [1] in Translation Project, but I noticed it is not
included to the release tarball. Should be po/pt_BR.po. Could this
please be taken care of?

[1] http://translationproject.org/PO-files/pt_BR/mailutils-3.3.pt_BR.po

Thanks in advance!

Best regards,
Rafael Fontenelle