Fix gcc 14 build issues

Origin: https://src.fedoraproject.org/rpms/mapnik/blob/rawhide/f/mapnik-gcc14.patch
Bug: mapnik/mapnik#4467


Gbp-Pq: Name gcc14.patch

include/mapnik/util/singleton.hpp

@@ -39,15 +39,15 @@
 namespace mapnik {
 
 template<typename T>
-class CreateUsingNew
+class MAPNIK_DECL CreateUsingNew
 {
   public:
     static T* create() { return new T; }
     static void destroy(T* obj) { delete obj; }
 };
 
 template<typename T>
-class CreateStatic
+class MAPNIK_DECL CreateStatic
 {
   private:
     using storage_type = typename std::aligned_storage<sizeof(T), alignof(T)>::type;