seastar: bump seastar to latest upstream master

for silencing GCC warnings

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

diff --git a/src/seastar b/src/seastar
index afafbaa8d43627fffa00541ac26a31904d0268ab..27b15a1972f08df8cdd2e939f39da5461bd8825e 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit afafbaa8d43627fffa00541ac26a31904d0268ab
+Subproject commit 27b15a1972f08df8cdd2e939f39da5461bd8825e