package/pistache: fix libressl build

Fix the following libressl build failure which is probably raised since
the addition of the package in commit
65d891ef:

../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



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>