Fix make release

Summary: Don't define if already defined.

Test Plan: Running make release in parallel, will not commit if it fails.

Reviewers: emayanke

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13833

tools/blob_store_bench.cc

@@ -11,7 +11,9 @@
 // BlobStore does costly asserts to make sure it's running correctly, which
 // significantly impacts benchmark runtime.
 // NDEBUG will compile out those asserts.
+#ifndef NDEBUG
 #define NDEBUG
+#endif
 
 using namespace rocksdb;
 using namespace std;