seastar: use the modernized cmake

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

diff --git a/src/seastar b/src/seastar
index 67bb775e7749158e4efe6edd5bad615998341ace..3dd9bab4cdf5c6e30e67bd40da8b41b56b9ff61f 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 67bb775e7749158e4efe6edd5bad615998341ace
+Subproject commit 3dd9bab4cdf5c6e30e67bd40da8b41b56b9ff61f