Update seastar submodule
Update seastar submodule

* seastar b5b2ee53d...4c7c5c7c4 (1):
  > Merge "add support for printing backtraces on one line" from Benny

Fixes #5464.

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