19634: tree-wide: mixed box of compilation fixes with clang r=benpicco a=maribu
### Contribution description
As the title says: This should increase the number of apps being able to build with clang quite a bit.
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@posteo.net>