add border-radius for el-loading root element

[VisualYuki]

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[pull-request-triage]

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[github-actions]

Hello @VisualYuki, thank you for contributing to element-plus, please see our guideline to see how to make contribution

[github-actions]

[VisualYuki]

playground example.
short example for my commit, uncomment 35-37 lines.
https://element-plus.run/#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

[github-actions]

🧪 Playground Preview: https://element-plus.run/?pr=16751
Please comment the example via this playground if needed.

[btea]

Thank you for your contribution! However, it may be more convenient and faster to adjust this requirement by yourself through style adjustment.