Enable -Wextra in CI builds

[igrr]

Related area

CI

Hardware specification

n/a

Is your feature request related to a problem?

As a follow-up to #6782 (comment), we should now add -Wextra when building the code in CI.

Describe the solution you'd like

• Option 1: something along the lines of Error even w/warnings disabled for no-return fcns esp8266/Arduino#8495 (move arguments into a file, use @ to tell GCC to read arguments from file)
• Option 2: In a CI build, use sed to add -Wextra to the list of warning flags in platform.txt

Describe alternatives you've considered

No response

Additional context

No response

I have checked existing list of Feature requests and the Contribution Guide

• I confirm I have checked existing list of Feature requests and Contribution Guide.

[Parsaabasi]

Hello,

This issue has beed fixed by previous releases. Therefore I close it. In case it persists, please feel free to reopen it.

Thanks.