Create waitfuncs.c for pg_isolation_test_session_is_blocked().

The next commit makes the function inspect an additional non-lock
contention source, so it no longer fits in lockfuncs.c.

Reviewed by Robert Haas.

Discussion: https://postgr.es/m/20240512232923.aa.nmisch@google.com

src/backend/utils/adt/meson.build

@@ -103,6 +103,7 @@ backend_sources += files(
   'varchar.c',
   'varlena.c',
   'version.c',
+  'waitfuncs.c',
   'windowfuncs.c',
   'xid.c',
   'xid8funcs.c',