update_engine: fixed warnings from cpplint

Fixed all the cpplint warnings in update engine.

BUG=None
TEST=Unit tests still pass.

Change-Id: I285ae858eec8abe0b26ff203b99a42a200ceb71c
Reviewed-on: https://chromium-review.googlesource.com/204027
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/update_attempter.h b/update_attempter.h
index 82cba8c..007da5b 100644
--- a/update_attempter.h
+++ b/update_attempter.h
@@ -10,6 +10,7 @@
 #include <memory>
 #include <string>
 #include <vector>
+#include <utility>
 
 #include <base/time/time.h>
 #include <glib.h>