Install wait_event_types.h in VPATH builds

An extra rule is needed in src/include/Makefile for VPATH builds to
install any generated server-side include files, and wait_event_types.h
was forgotten from the set.

Issue introduced by fa88928.

Reported-by: Christoph Berg
Discussion: https://postgr.es/m/ZTAA11u7CtX6NqlK@msg.df7cb.de

src/include/Makefile

@@ -54,7 +54,7 @@ install: all installdirs
 	  $(INSTALL_DATA) $(srcdir)/$$dir/*.h '$(DESTDIR)$(includedir_server)'/$$dir || exit; \
 	done
 ifeq ($(vpath_build),yes)
-	for file in catalog/schemapg.h catalog/system_fk_info.h catalog/pg_*_d.h storage/lwlocknames.h utils/probes.h; do \
+	for file in catalog/schemapg.h catalog/system_fk_info.h catalog/pg_*_d.h storage/lwlocknames.h utils/probes.h utils/wait_event_types.h; do \
 	  $(INSTALL_DATA) $$file '$(DESTDIR)$(includedir_server)'/$$file || exit; \
 	done
 endif