rocksdb: ignore m4

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

diff --git a/src/rocksdb b/src/rocksdb
index a5e1ec9bcddc44038a735e269fd5d6586f01c2cb..dcdb0dd29232ece43f093c99220b0eea7ead51ff 160000 (submodule)
--- a/src/rocksdb
+++ b/src/rocksdb
@@ -1 +1 @@
-Subproject commit a5e1ec9bcddc44038a735e269fd5d6586f01c2cb
+Subproject commit dcdb0dd29232ece43f093c99220b0eea7ead51ff