dnsdist: Fix FTBFS with GCC 11

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>

src/patches/dnsdist-1.6.0-missing-mutex-header.patch

@@ -0,0 +1,22 @@
+From 65bb0ab32a449d8fb8cf9a4802d16ba4912d9fc4 Mon Sep 17 00:00:00 2001
+From: Remi Gacogne <remi.gacogne@powerdns.com>
+Date: Tue, 25 May 2021 10:09:43 +0200
+Subject: [PATCH] dnsdist-1.6.x: Backport a missing mutex header
+
+---
+ pdns/lock.hh | 2 ++
+ 1 file changed, 2 insertions(+)
+
+diff --git a/pdns/lock.hh b/pdns/lock.hh
+index 09299d7979d..0e88118c564 100644
+--- a/pdns/lock.hh
++++ b/pdns/lock.hh
+@@ -20,6 +20,8 @@
+  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+  */
+ #pragma once
++
++#include <mutex>
+ #include <shared_mutex>
+ 
+ class ReadWriteLock