Clean up newly added guc_tables.inc.c

There was a missing makefile rule to clean up the guc_tables.inc.c
symlink in src/include/.  Oversight in commit 6359989654.

Author: Nathan Bossart <nathandbossart@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/dae6fe89-1e0c-4c3f-8d92-19d23374fb10%40eisentraut.org

src/include/Makefile

@@ -72,7 +72,7 @@ uninstall:
 
 
 clean:
-	rm -f utils/fmgroids.h utils/fmgrprotos.h utils/errcodes.h utils/header-stamp
+	rm -f utils/fmgroids.h utils/fmgrprotos.h utils/guc_tables.inc.c utils/errcodes.h utils/header-stamp
 	rm -f storage/lwlocknames.h utils/probes.h utils/wait_event_types.h
 	rm -f nodes/nodetags.h nodes/header-stamp
 	$(MAKE) -C catalog clean