Makefile: update submodule reference for leveldb.

diff --git a/src/leveldb b/src/leveldb
index f2d406b4213a22bfed3acfdb071fbec9ca9205cc..e9d7916ce652bfaed334106f4bf83f4d65f5c66e 160000 (submodule)
--- a/src/leveldb
+++ b/src/leveldb
@@ -1 +1 @@
-Subproject commit f2d406b4213a22bfed3acfdb071fbec9ca9205cc
+Subproject commit e9d7916ce652bfaed334106f4bf83f4d65f5c66e