seastar: bump seastar to latest upstream master

to pick up the change to drop protobuf dependency

Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/src/seastar b/src/seastar
index 2e3c6da3f8c8b69cb790b056661717f1d7346a61..8ed9771ae98130d5adbccdaea4ecbac738170939 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 2e3c6da3f8c8b69cb790b056661717f1d7346a61
+Subproject commit 8ed9771ae98130d5adbccdaea4ecbac738170939