https://bugs.gentoo.org/790719
https://github.com/hhoffstaette/portage/blob/master/net-dns/dnsdist/files/dnsdist-1.6.0-add-missing-include.patch

From: Kees Monshouwer <mind04@monshouwer.org>
Date: Tue, 11 May 2021 10:27:01 +0200
Subject: [PATCH] add missing includes (Fedora 34, gcc 11.1 / clang 12)

[HH: reduced to necessary patch for dnsdist]
---
 lock.hh      | 1 +
 1 file changed, 1 insertions(+)

diff --git a/lock.hh b/lock.hh
index 09299d7979d..0644502f871 100644
--- a/lock.hh
+++ b/lock.hh
@@ -20,6 +20,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 #pragma once
+#include <mutex>
 #include <shared_mutex>
 
 class ReadWriteLock