seastar: lower the required yaml-cpp version to 0.5.1

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

diff --git a/src/seastar b/src/seastar
index 3dd9bab4cdf5c6e30e67bd40da8b41b56b9ff61f..de0a2715028bf1931e6730ab289b5daec3bdebb9 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 3dd9bab4cdf5c6e30e67bd40da8b41b56b9ff61f
+Subproject commit de0a2715028bf1931e6730ab289b5daec3bdebb9