cmake: bump seastar version

$ git shortlog 5531940ecd6fa6e5a30bbce8e0573db6f721d343 ^2a9504b3238cba4150be59353bf8d0b3a01fe39c
Evgeny Lazin (1):
      Use tls socket to retrieve distinguished name

Noah Watkins (1):
      Merge pull request redpanda-data#21 from Lazin/feature/padd-dn-through-callback

Travis Downs (3):
      Add a message in verbose when tests pass
      Fix escaping of regexes
      Decode kernel backtraces in seastar-addr2line

cmake/oss.cmake.in

@@ -164,7 +164,7 @@ ExternalProject_Add(cryptopp
 
 ExternalProject_Add(seastar
   GIT_REPOSITORY https://github.com/redpanda-data/seastar.git
-  GIT_TAG 2a9504b3238cba4150be59353bf8d0b3a01fe39c
+  GIT_TAG 5531940ecd6fa6e5a30bbce8e0573db6f721d343
   INSTALL_DIR    @REDPANDA_DEPS_INSTALL_DIR@
   CMAKE_COMMAND ${CMAKE_COMMAND} -E env ${cmake_build_env} ${CMAKE_COMMAND}
   CMAKE_ARGS