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.

[CLAassistant]

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[gkodinov]

Hello and thank you for taking the effort to contribute a fix to this project.

Can you please sort out the CLA agreement?

Also, for future reference, please consider creating a JIRA (or working on an existing one) going forward. I am going to create one for you here once the CLA is signed, so you won't have to worry about this one.

[gkodinov]

Please figure out the CLA agreement.