Update mpack1.h

3rd/msgpack/include/mpack1.h

@@ -3,10 +3,18 @@
 #include "mpack.h"
 #include "pocketpy.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef bool (*is_mpack_ext_t)(py_Type type);
 typedef bool (*py_to_mpack_ext_t)(py_Ref object, mpack_writer_t* writer);
 typedef bool (*mpack_to_py_ext_t)(py_StackRef tmp, mpack_node_t node);
 
 void mpack_config_exttype_callbacks(is_mpack_ext_t is_ext,
                                     py_to_mpack_ext_t to_mpack,
-                                    mpack_to_py_ext_t to_py);
+                                    mpack_to_py_ext_t to_py);
+
+#ifdef __cplusplus
+}
+#endif