[Debian-med-packaging] Added symbols file to libbpp-seq

[Andreas Tille]

Hi Julien,

since I just added a symbols file to libbpp-core (see #890400) I think
it make sense to provide this for all libbpp-* libraries to make sure
we'll spot ABI changes right in time.  I added a symbols file also for
libbpp-seq by strictly following the description in wiki[1].  Please
let me know if you need help for the other packages.

Kind regards

      Andreas.

[1] https://wiki.debian.org/UsingSymbolsFiles

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