Move a fall through comment

When compiling with --strict-warnings using gcc 7.4.0 the compiler
complains that a case falls through, even though there is an explicit
comment stating this. Moving the comment outside of the conditional
compilation section resolves this.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9131)

(cherry picked from commit a2e52044)