pg_validatebackup: Fix 'make clean' to remove tmp_check.

Report by Tom Lane.

Discussion: http://postgr.es/m/22394.1585951968@sss.pgh.pa.us

src/bin/pg_validatebackup/Makefile

@@ -31,6 +31,7 @@ uninstall:
 
 clean distclean maintainer-clean:
 	rm -f pg_validatebackup$(X) $(OBJS)
+	rm -rf tmp_check
 
 check:
 	$(prove_check)