Add ChangeLog entry for #3217: avoid re-assignment

Signed-off-by: Janos Follath <janos.follath@arm.com>
diff --git a/ChangeLog.d/remove-extra-assignment.txt b/ChangeLog.d/remove-extra-assignment.txt
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+Changes
+   * Remove superfluous assignment in mbedtls_ssl_parse_certificate(). Reported
+     in #3182 and fix submitted by irwir. #3217