src/server/listener.cc: fix libressl build

[ffontaine]

Fix the following libressl build failure:

../src/server/listener.cc: In member function 'void Pistache::Tcp::Listener::setupSSLAuth(const std::string&, const std::string&, int (*)(int, void*))': ../src/server/listener.cc:582:29: error: 'SSL_verify_cb' was not declared in this scope; did you mean 'RSA_verify'?
  582 |                            (SSL_verify_cb)cb
      |                             ^~~~~~~~~~~~~
      |                             RSA_verify

Fixes:

• http://autobuild.buildroot.org/results/066fc078980e5216f38411eee455088e15fa1101

[codecov-commenter]

Codecov Report

Base: 78.60% // Head: 78.58% // Decreases project coverage by -0.02% :warning:

Coverage data is based on head (55e5c2f) compared to base (294bcb9). Patch has no changes to coverable lines.

:exclamation: Current head 55e5c2f differs from pull request most recent head 656bff2. Consider uploading reports for the commit 656bff2 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1124 +/- ## ========================================== - Coverage 78.60% 78.58% -0.02% ========================================== Files 53 53 Lines 6879 6879 ========================================== - Hits 5407 5406 -1 - Misses 1472 1473 +1 ``` | [Impacted Files](https://codecov.io/gh/pistacheio/pistache/pull/1124?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/server/listener.cc](https://codecov.io/gh/pistacheio/pistache/pull/1124?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL3NlcnZlci9saXN0ZW5lci5jYw==) | `71.11% <ø> (ø)` | | | [include/pistache/async.h](https://codecov.io/gh/pistacheio/pistache/pull/1124?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-aW5jbHVkZS9waXN0YWNoZS9hc3luYy5o) | `89.94% <0.00%> (-0.19%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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[kiplingw]

LGTM. @dennisjenkins75 and @Tachi107?

[Tachi107]

@ffontaine thanks for the patch! Looks good to me as well

Edit: removed the security label as this doesn't fix a security issue, unlike #1104 for example - we don't have a policy on this, but I thought it'd make sense