seastar: pickup the change to link pthread

otherwise seastar applications might fail to link on ubuntu xenial due
to unresolvable symbols like thread_setname_np()

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

diff --git a/src/seastar b/src/seastar
index 6f5dcc44147f60bf2abe04581da7a4cca33eaa08..4ad64a507b61056afdab1d24c13c0a94817833e2 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 6f5dcc44147f60bf2abe04581da7a4cca33eaa08
+Subproject commit 4ad64a507b61056afdab1d24c13c0a94817833e2