rocksdb: fix 32-bit build

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

diff --git a/src/rocksdb b/src/rocksdb
index f371049d895a083810746a00049688043d8cea7e..15ed8c97fdbdf022ec9c8da02b06f4cd9189bb7e 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit f371049d895a083810746a00049688043d8cea7e
+Subproject commit 15ed8c97fdbdf022ec9c8da02b06f4cd9189bb7e