Fix compilation with gcc-15

Bug: https://bugs.gentoo.org/937628
Bug: https://github.com/PowerDNS/pdns/issues/14549

Signed-off-by: Holger Hoffstätte <holger@applied-asynchrony.com>

diff -rup dnsdist-1.9.6/ext/json11/json11.cpp dnsdist-1.9.6-gcc15/ext/json11/json11.cpp
--- dnsdist-1.9.6/ext/json11/json11.cpp	2024-07-15 11:46:15.000000000 +0200
+++ dnsdist-1.9.6-gcc15/ext/json11/json11.cpp	2024-08-09 18:03:51.090140981 +0200
@@ -22,6 +22,7 @@
 #include "json11.hpp"
 #include <cassert>
 #include <cmath>
+#include <cstdint>
 #include <cstdlib>
 #include <cstdio>
 #include <limits>