pg_plan_advice: Always install pg_plan_advice.h, and in the right place

pg_plan_advice: Always install pg_plan_advice.h, and in the right place

The Makefile failed to set HEADERS_pg_plan_advice, so the header wasn't
installed. Fixing that reveals another problem: since this is just a
loadable module, not an extension, the header file is installed into
$(includedir_server)/contrib rather than $(includedir_server)/extension.
While we have no existing cases of installing header files there, it
appears to be the intent of pgxs.mk.  However, this is inconsistent with
meson.build, which was using dir_include_extension. Changing that to
dir_include_server / 'contrib' makes the install locations consistent
across the two builds.

Author: Zsolt Parragi <zsolt.parragi@percona.com>
Discussion: http://postgr.es/m/CAN4CZFP6NOjv__4Mx+iQD8StdpbHvzDAatEQn2n15UKJ=MySSQ@mail.gmail.com