Fix includes for clang on OS X

Summary:
Fix below error:

  use of undeclared identifier 'errno'

Test Plan: doitlive

Reviewers: IslamAbdelRahman, sdong

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D57849

util/env_chroot.cc

@@ -7,6 +7,8 @@
 
 #include "util/env_chroot.h"
 
+#include <errno.h>
+#include <string.h>
 #include <unistd.h>
 
 #include <string>