Add missing include.

libc++ cleaned up a leaky header.

Change-Id: I5ac0f43a075257a9138a82be379c9cffe96e0ab9
diff --git a/benchmarks/time_benchmark.cpp b/benchmarks/time_benchmark.cpp
index 6688bbc..1b0d08d 100644
--- a/benchmarks/time_benchmark.cpp
+++ b/benchmarks/time_benchmark.cpp
@@ -17,6 +17,7 @@
 #include <sys/syscall.h>
 #include <sys/time.h>
 #include <time.h>
+#include <unistd.h>
 
 #include <benchmark/Benchmark.h>