Fixed a compile error in util/arena.h

Summary:
Fixed the compile error in util/arena.h caused by not
including TLB related header.

Test Plan: make db_stress

Reviewers: igor, sdong, anthony, IslamAbdelRahman

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D47697

util/arena.h

@@ -12,6 +12,9 @@
 // size, it uses malloc to directly get the requested size.
 
 #pragma once
+#ifndef OS_WIN
+#include <sys/mman.h>
+#endif
 #include <cstddef>
 #include <cerrno>
 #include <vector>