Provide for contrib and pgxs modules to install include files.

This allows out-of-tree PLs and similar code to get access to
definitions needed to work with extension data types.

The following existing modules now install headers: contrib/cube,
contrib/hstore, contrib/isn, contrib/ltree, contrib/seg.

Discussion: https://postgr.es/m/87y3euomjh.fsf%40news-spur.riddles.org.uk