Fix unittest compile

Bug: 26955860
TEST=cros_workon_make --test update_engine runs with no problem.

Change-Id: I8821d205f87ead8485b9f23186d4fce78519d0b4
diff --git a/payload_consumer/extent_writer_unittest.cc b/payload_consumer/extent_writer_unittest.cc
index 4d6b4d6..24d238e 100644
--- a/payload_consumer/extent_writer_unittest.cc
+++ b/payload_consumer/extent_writer_unittest.cc
@@ -16,6 +16,8 @@
 
 #include "update_engine/payload_consumer/extent_writer.h"
 
+#include <fcntl.h>
+
 #include <algorithm>
 #include <string>
 #include <vector>