cmake: update seastar to tip of redpanda-data/seastar/master

This picks up the commit of 21d665aebf7b74371e4527299441812de5eea6c2,
which instantiate the template specialization of
template future<> sleep_abortable<manual_clock>(typename manual_clock::duration, abort_source&)
which is used by
net::connection_rate<seastar::manual_clock>::spawn_updating_fiber_if_needed()
in src/v/net/connection_rate.h

without this change, we'd run into following FTBFS:

src/v/net/tests/CMakeFiles/net_connection_rate_rpunit.dir/connection_rate_test.cc.o: in function `net::connection_rate<seastar::manual_clock>::spawn_updating_fiber_if_needed(seastar::lw_shared_ptr<net::connection_rate_counter<seastar::manual_clock> >)::{lambda()redpanda-data#1}::operator()()':
connection_rate_test.cc:(.text._ZZN3net15connection_rateIN7seastar12manual_clockEE30spawn_updating_fiber_if_neededENS1_13lw_shared_ptrINS_23connection_rate_counterIS2_EEEEENUlvE_clEv[_ZZN3net15connection_rateIN7seastar12manual_clockEE30spawn_updating_fiber_if_neededENS1_13lw_shared_ptrINS_23connection_rate_counterIS2_EEEEENUlvE_clEv]+0x271): undefined reference to `seastar::future<void> seastar::sleep_abortable<seastar::manual_clock>(seastar::manual_clock::duration, seastar::abort_source&)'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Signed-off-by: Kefu Chai <[email protected]>

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 f8ec733c36f0829d56a17103c916154a946128be
+  GIT_TAG 2a9504b3238cba4150be59353bf8d0b3a01fe39c
   INSTALL_DIR    @REDPANDA_DEPS_INSTALL_DIR@
   CMAKE_COMMAND ${CMAKE_COMMAND} -E env ${cmake_build_env} ${CMAKE_COMMAND}
   CMAKE_ARGS