Changelog entry for benchmark improvement

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/benchmark-ecdh.txt b/ChangeLog.d/benchmark-ecdh.txt
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+Features
+   * The benchmark program now reports times for both ephemeral and static
+     ECDH in all ECDH configurations.