nebula-storage make werror

[tongheihei]

Description:I cloned the nebula-storage and nebula-common,and prepare the environment as the doc prepare resource mentioned.The doc tells that the gcc version should be 7.1.0 or higher,and follow the doc,I installed gcc 7.5.0 on my computer.But error occured as above. After I update my gcc version to gcc 9.2.0 ,the error solved.So is that a bug or the doc should change the minimum gcc version?