fstrm/libfstrm.sym: Add tcp_writer symbols
fstrm/libfstrm.sym: Add tcp_writer symbols

This commit exports the new public tcp_writer functions introduced in
the previous commit.

I've guessed that the next feature release will be 'fstrm 0.4.0'. This
file should be updated with the right version if that turns out not to
be the case.

farsightsec/fstrm,farsightsec/fstrm