rocksdb: sync with upstream

to avoid warning with glibc 2.25, see
https://github.com/facebook/rocksdb/pull/2208 and
https://github.com/ceph/rocksdb/pull/17

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

diff --git a/src/rocksdb b/src/rocksdb
index 9f2cc59ec5065c828dabbab483506bde681c955c..e15382c09c87a65eaeca9bda233bab503f1e5772 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit 9f2cc59ec5065c828dabbab483506bde681c955c
+Subproject commit e15382c09c87a65eaeca9bda233bab503f1e5772