Make config.h check for unistd.h inclusion before leveldb includes it

src/core/disk_ops/disk_leveldb.cpp

@@ -17,6 +17,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <errno.h>
+#include <common/config.h>
 #include <leveldb/db.h>
 #include <leveldb/options.h>