libchromeos: Include numeric for std::iota.

BUG=None
TEST=cros_run_unit_tests --board=gizmo libchromeos.

Change-Id: I9fb3b8ce6c6f5938e39fb1bd7ede1ce2a18c9fec
Reviewed-on: https://chromium-review.googlesource.com/285850
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Trybot-Ready: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/chromeos/data_encoding_unittest.cc b/chromeos/data_encoding_unittest.cc
index 549c478..70bd748 100644
--- a/chromeos/data_encoding_unittest.cc
+++ b/chromeos/data_encoding_unittest.cc
@@ -5,6 +5,7 @@
 #include <chromeos/data_encoding.h>
 
 #include <algorithm>
+#include <numeric>
 
 #include <gtest/gtest.h>