sstables: remove circular reference
sstables: remove circular reference

writer.hh includes sstables.hh which includes writer.hh
We can't remove the reference if we include core/fstream.hh into writer.hh instead

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@cloudius-systems.com>

avikivity/scylla,stamhe/scylla,scylladb/scylla,rentongzhang/scylla,phonkee/scylla,phonkee/scylla,dwdm/scylla,aruanruan/scylla,raphaelsc/scylla,justintung/scylla,gwicke/scylla,avikivity/scylla,wildinto/scylla,tempbottle/scylla,aruanruan/scylla,shaunstanislaus/scylla,acbellini/scylla,dwdm/scylla,linearregression/scylla,eklitzke/scylla,rentongzhang/scylla,kangkot/scylla,asias/scylla,kangkot/scylla,bowlofstew/scylla,eklitzke/scylla,guiquanz/scylla,avikivity/scylla,rluta/scylla,justintung/scylla,tempbottle/scylla,glommer/scylla,victorbriz/scylla,senseb/scylla,raphaelsc/scylla,duarten/scylla,tempbottle/scylla,kangkot/scylla,kjniemi/scylla,eklitzke/scylla,kjniemi/scylla,rentongzhang/scylla,scylladb/scylla,glommer/scylla,linearregression/scylla,gwicke/scylla,rluta/scylla,linearregression/scylla,duarten/scylla,senseb/scylla,capturePointer/scylla,glommer/scylla,stamhe/scylla,victorbriz/scylla,capturePointer/scylla,phonkee/scylla,shaunstanislaus/scylla,asias/scylla,raphaelsc/scylla,dwdm/scylla,asias/scylla,capturePointer/scylla,justintung/scylla,guiquanz/scylla,wildinto/scylla,senseb/scylla,acbellini/scylla,acbellini/scylla,respu/scylla,gwicke/scylla,victorbriz/scylla,shaunstanislaus/scylla,guiquanz/scylla,scylladb/scylla,kjniemi/scylla,rluta/scylla,stamhe/scylla,bowlofstew/scylla,duarten/scylla,respu/scylla,wildinto/scylla,respu/scylla,scylladb/scylla,aruanruan/scylla,bowlofstew/scylla