PG15 removes the utils/int8.h header

Removing it on the older majors doesn't seem to hurt, so do it unconditionally.

--- a/src/backend/slony1_funcs.c
+++ b/src/backend/slony1_funcs.c
@@ -44,7 +44,7 @@
 #include "utils/memutils.h"
 #include "utils/hsearch.h"
 #include "utils/timestamp.h"
-#include "utils/int8.h"
+//#include "utils/int8.h"
 #ifdef HAVE_GETACTIVESNAPSHOT
 #include "utils/snapmgr.h"
 #endif
