Fix build with boost 1.83

Index: include/mapnik/json/generic_json.hpp
--- include/mapnik/json/generic_json.hpp.orig
+++ include/mapnik/json/generic_json.hpp
@@ -32,6 +32,7 @@
 #include <boost/spirit/include/qi.hpp>
 #include <boost/spirit/include/phoenix.hpp>
 #include <boost/fusion/include/std_pair.hpp>
+#include <boost/regex/v5/icu.hpp>
 #pragma GCC diagnostic pop
 
 #include <vector>
