Export GPG_TTY environment variable from profile

See `man gpg-agent` for details:

    You should always add the following lines to your .bashrc or
    whatever initialization file is used for all shell
    invocations:

    GPG_TTY=$(tty)
    export GPG_TTY

    It is important that this environment variable always reflects
    the output of the tty command.  For W32 systems this option is
    not required.