include c header file in sdict

That means hackage sdist is broken. I guess we should add CI for this.

added typebug label

mentioned in issue #399

merged

I just noticed there's another issue that makes ghcup hard to build from hackage:

package streamly
  flags: +use-unliftio

I couldn't make it build without unliftio. Also see https://github.com/composewell/streamly/pull/1485#issuecomment-1127532793