No longer install module API headers

No longer install module API headers

Various header files providing an API to implement own cache and
object types or implement link types have been available in the
past. Their API is marked unstable in the documentation and nobody
is using them so far.

Technically they have been causing ABI breakages in at least two
3.2.x releases. In order to avoid future accidental breakage, no
longer install these and require module developers to clone the
git tree and build modules off the git tree directly to make sure
that the API is unstable at this point.

Signed-off-by: Thomas Graf <tgraf@suug.ch>