stub implement pseudoheader, now required by mirage-types

[hannesm]

@yomimono this makes the tests compile (for some reason the Dns_resolver_unix::resolution-error test fails on my FreeBSD)

[hannesm]

nearly all builds completed successfully; only some alpine-3.3 couldn't compile lwt...

[samoht]

yes the alpine error is because they lwt configuration script needs ncurse which is not installed by default there ... @avsm and I tracked that issue yesterday, should be fixed soon.

[yomimono]

Thanks for this @hannesm :) I've restarted the tests; we'll see how they do.

[hannesm]

there was only a single failing build, you can restart individual builds... all others were fine

[yomimono]

Ah, thanks, I didn't realize that - barely noticed the "restart" button on the main page, let alone within the individual builds. I'll waste less compute cycles next time.

[hannesm]

(and the alpine build is unlikely to be working before something like ocaml/opam-repository#6791 gets merged)

[hannesm]

same as yesterday, IMHO this can safely be merged since all other platforms are happy (and it fixes make test with mirage-types master).

[hannesm]

merged this minor fix (won't fix your alpine opam troubles, but there are several issues in the opam repository)