Fix error reporting in create_libaio

[andreas-schwab]

The io_setup function in libaio returns a negated errno value on error,
but strerror expects a normal errno value.

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[gemini-code-assist]

Code Review

This pull request modifies tpool/aio_libaio.cc to pass the negated return value of io_setup to strerror when printing a warning message. Since io_setup returns a negative error code on failure, negating it ensures strerror receives the correct positive error number. There are no review comments, and I have no additional feedback to provide.

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