rocksdb: bump up to v6.15.5

to pick up a bunch of cleanups which silences warnings like range-based
loop using `auto` instead of `auto &`.

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

diff --git a/src/rocksdb b/src/rocksdb
index f86d378860f399bf595bd1135f39f03a9eaca59f..c540de6f709b66efd41436694f72d6f7986a325b 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit f86d378860f399bf595bd1135f39f03a9eaca59f
+Subproject commit c540de6f709b66efd41436694f72d6f7986a325b