Add changelog entry

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/ChangeLog.d/add_final_delay_accessor b/ChangeLog.d/add_final_delay_accessor
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+++ b/ChangeLog.d/add_final_delay_accessor
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+Features
+   * Add the function mbedtls_timing_get_final_delay() to access the private
+     final delay field in an mbedtls_timing_delay_context, as requested in
+     #5183