Include <unistd.h> in db_test

Summary: This is the only compile issue in Ubuntu. It might be better to include <unistd.h> only in env_posix and add Truncate function to Env, but since we use truncate only in db_test, I don't think it makes much sense.

Test Plan: Rocksdb now compiles on Ubuntu!

Reviewers: dhruba, kailiu

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14127

db/db_test.cc

@@ -9,6 +9,7 @@
 
 #include <algorithm>
 #include <set>
+#include <unistd.h>
 
 #include "rocksdb/db.h"
 #include "rocksdb/filter_policy.h"