seastar: pick up fixes to address build failures

see also https://groups.google.com/forum/#!topic/seastar-dev/uEMYGgYJKOY

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

diff --git a/src/seastar b/src/seastar
index 9c32e23e722da580e31462e11c48eba3f6a97a7f..8188d3b8b7091db9227824791d022d64c98193e9 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 9c32e23e722da580e31462e11c48eba3f6a97a7f
+Subproject commit 8188d3b8b7091db9227824791d022d64c98193e9