Don't include wait_event.h in pgstat.h

Don't include wait_event.h in pgstat.h

wait_event.h itself includes wait_event_types.h, which is a generated
file, so it's nice that we can avoid compiling >10% of the tree just
because that file is regenerated.

To avoid breaking too many third-party modules, we now #include
utils/wait_classes.h in storage/latch.h.  Then, the very common case
of doing
	WaitLatch(..., PG_WAIT_EXTENSION)
continues to work by including just storage/latch.h.  (I didn't try to
determine how many modules would actually break if we don't do this, but
this seems a convenient and low-impact measure.)

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/202602181214.gcmhx2vhlxzp@alvherre.pgsql