NaCl: Add missing #include of <unistd.h> to get sysconf()

NaCl: Ensure that sysconf() is declared: add missing #include of <unistd.h>

This is necessary to roll in a new NaCl revision after I committed a
NaCl-side change which cleans up the service_runtime/include/ headers.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=3909
TEST= build

Committed: https://crrev.com/d5aa153518373e68f92e42fc702f51dd13d06889
Cr-Commit-Position: refs/heads/master@{#300593}

[Mark Seaborn, 2014-10-21 22:03:37]

mseaborn@chromium.org changed reviewers:
+ teravest@chromium.org

[teravest, 2014-10-21 22:10:59]

lgtm

[Mark Seaborn, 2014-10-21 23:01:21]

The CQ bit was checked by mseaborn@chromium.org

[commit-bot: I haz the power, 2014-10-21 23:02:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/651643007/1

[commit-bot: I haz the power, 2014-10-22 01:08:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-22 04:04:32]

Patchset 1 (id:??) landed as
https://crrev.com/d5aa153518373e68f92e42fc702f51dd13d06889
Cr-Commit-Position: refs/heads/master@{#300593}