rocksdb: fix 32-bit build

Signed-off-by: Sage Weil <sage@redhat.com>

diff --git a/src/rocksdb b/src/rocksdb
index df5f727d2952a3f298250c7d19991077ecb39701..6ca7befb55767784a447a5daddd09e387ec92bd3 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit df5f727d2952a3f298250c7d19991077ecb39701
+Subproject commit 6ca7befb55767784a447a5daddd09e387ec92bd3