Add missing include.

libc++ used to leak unistd.h from every headers. This is fixed in the
latest version.

The code that needs this is not upstream code
(https://googleplex-android-review.git.corp.google.com/#/c/645357/),
so no need to send upstream.

Change-Id: I51111c08b4d2620a5d264e20646e6606a3f39c7e
(cherry picked from commit 8fc83651c5d7c5bc7413be76f82df0b23c681313)
diff --git a/sgdisk.cc b/sgdisk.cc
index 2d89a0e..1892e2d 100644
--- a/sgdisk.cc
+++ b/sgdisk.cc
@@ -18,6 +18,7 @@
 #include <errno.h>
 #include "gptcl.h"
 #include <fcntl.h>
+#include <unistd.h>
 
 using namespace std;