Fix TSAN build error of D36447

Summary:
D36447 caused build error when using COMPILE_WITH_TSAN=1.
This diff fixes the error.

Test Plan: jenkins

Reviewers: igor, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D36579

db/db_bench.cc

@@ -24,6 +24,7 @@ int main() {
 #include <numaif.h>
 #endif
 
+#include <unistd.h>
 #include <fcntl.h>
 #include <inttypes.h>
 #include <cstddef>