Fixed gcc-4.7 building problem for update_engine (again).

TEST=Built using gcc-4.7 for lumpy.
BUG=None

Change-Id: Ic5dc527044d75549d7a125c64990bbee4d2dfa21
Reviewed-on: https://gerrit.chromium.org/gerrit/26138
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Ready: Han Shen <shenhan@chromium.org>
Tested-by: Han Shen <shenhan@chromium.org>
diff --git a/omaha_hash_calculator.h b/omaha_hash_calculator.h
index 9735ac8..784326a 100644
--- a/omaha_hash_calculator.h
+++ b/omaha_hash_calculator.h
@@ -6,6 +6,7 @@
 #define CHROMEOS_PLATFORM_UPDATE_ENGINE_OMAHA_HASH_CALCULATOR_H__
 
 #include <string>
+#include <unistd.h>
 #include <vector>
 
 #include <openssl/sha.h>