only expose save_to_cairo_file if cairo support available

include/mapnik/image_util.hpp

@@ -58,9 +58,11 @@ public:
     }
 };
 
+#if defined(HAVE_CAIRO)
 MAPNIK_DECL void save_to_cairo_file(mapnik::Map const& map,
                                     std::string const& filename,
                                     std::string const& type);
+#endif
 
 template <typename T>
 MAPNIK_DECL void save_to_file(T const& image,