pgsql: Remove circular #include's between wait_event.h and wait_event_t

Remove circular #include's between wait_event.h and wait_event_types.h

wait_event_types.h is generated by the code, and included wait_event.h.
wait_event.h did the opposite move, including wait_event_types.h,
causing a circular dependency between both.

wait_event_types.h only needs to now about the wait event classes, so
this information is moved into its own file, and wait_event_types.h uses
this new header so as it does not depend anymore on wait_event.h.

Note that such errors can be found with clang-tidy, with commands like
this one:
clang-tidy source_file.c --checks=misc-header-include-cycle -- \
-I/install/path/include/ -I/install/path/include/server/

Issue introduced by fa88928470b5.

Author: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Discussion: https://postgr.es/m/350192.1745768770@sss.pgh.pa.us

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b225c5e76ed1053e505e392423b0dab065a3b813

Modified Files
--------------
.../utils/activity/generate-wait_event_types.pl | 2 +-
src/include/utils/wait_classes.h | 29 ++++++++++++++++++++++
src/include/utils/wait_event.h | 16 ------------
3 files changed, 30 insertions(+), 17 deletions(-)