Update seastar submodule
Update seastar submodule

* seastar 12f18ce...5712816 (6):
  > tests: add signal_test to test list
  > Merge "Enhancements for memory_output_stream" from Paweł
  > seastar-addr2line: don't print an empty line between backtrace lines
  > seastar-addr2line: add --verbose option
  > seastar-addr2line: make prefix matching non-greedy
  > future: make available() const

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla