add missing include

Author: artemp

src/mapnik_map.cpp

@@ -35,6 +35,7 @@
 #include <mapnik/rule.hpp>
 #include <mapnik/layer.hpp>
 #include <mapnik/map.hpp>
+#include <mapnik/featureset.hpp>
 #include <mapnik/projection.hpp>
 #include <mapnik/view_transform.hpp>
 #include <mapnik/feature_type_style.hpp>