Add missing LLEvents:once to lua_types xml

[WolfGangS]

Description

Adds the LLEvents:once method that was missing from the lua types xml

A matching pr to the vscode plugin has been made secondlife/sl-vscode-plugin#18

Related Issues

n/a

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Checklist

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• All new and existing tests pass.
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• Documentation has been updated if needed.
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Additional Notes

As with #5022 #5015 this should have no impact on the viewer itself.