dnsdist: Fix our `meson` build not reaching a stable state

dnsdist: Fix our `meson` build not reaching a stable state

Before this commit calling `meson build` a second time right after building
triggered a re-generation of the `dnsdist-configuration-yaml-items-generated.cc`
file because `meson` wants it to be present in the build directory,
but we also want to generate it in the source directory so that it
can be included in the repository.
The current solution is a bit dirty, but our options are limited
because some files are only generated when YAML support is available,
because they require Rust, and we need them to be in the dist tarball.
Otto suggested that we could generate them only when building the dist
tarball, I'll look into it in a bit.