Fix doxygen warning about parameter name

Fix doxygen warning about parameter name

Before:

```
$ doxygen doc/Doxyfile
/Users/matthias/prog/fuser/libfuse/include/fuse.h:934: warning: argument 'private_data' of command @param is not found in the argument list of fuse_new_31(struct fuse_args *args, const struct fuse_operations *op, size_t op_size, void *user_data)
/Users/matthias/prog/fuser/libfuse/include/fuse.h:934: warning: The following parameter of fuse_new_31(struct fuse_args *args, const struct fuse_operations *op, size_t op_size, void *user_data) is not documented:
  parameter 'user_data'
```