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/delta_performer.h b/delta_performer.h
index 8fffffb..15c8af3 100644
--- a/delta_performer.h
+++ b/delta_performer.h
@@ -7,6 +7,7 @@
 
 #include <inttypes.h>
 
+#include <string>
 #include <vector>
 
 #include <base/time/time.h>