Adapt forward.jinja to check if compile has C or Cpp.

aidge_core/export_utils/templates/forward_header.jinja

 #ifndef DNN_HPP
 #define DNN_HPP
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 {#- For libraries #}
 {% for lib in libraries %}
 #include <{{ lib }}>
@@ -10,4 +14,8 @@
 {{ func }};
 {% endfor %}
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* DNN_HPP */