Fix the compilation error in flashcache.cc on Mac

Summary:
Fix the following compilation error in flashcache.cc on Mac

Undefined symbols for architecture x86_64:

"rocksdb::NewFlashcacheAwareEnv(rocksdb::Env*, int)", referenced from:
    rocksdb::Benchmark::Open(rocksdb::Options*) in db_bench.o

Test Plan: make db_bench

Reviewers: sdong, igor, rven

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D36657

utilities/flashcache/flashcache.cc

@@ -125,7 +125,7 @@ int FlashcacheWhitelistCurrentThread(Env* flashcache_aware_env) {
 
 #else   // !defined(ROCKSDB_LITE) && defined(OS_LINUX)
 std::unique_ptr<Env> NewFlashcacheAwareEnv(Env* base,
-                                           const std::string& flashcache_dev) {
+                                           const int cachedev_fd) {
   return nullptr;
 }
 int FlashcacheBlacklistCurrentThread(Env* flashcache_aware_env) { return -1; }