pg_combinebackup didn't clean its tmp_check directory, either.

Another oversight in dc2123400, visible when building/testing
in the source directory.  (There's a lot of stuff we could
simplify if we stop supporting that case, but for now it's
still mainstream.)

src/bin/pg_combinebackup/Makefile

@@ -44,6 +44,7 @@ uninstall:
 
 clean distclean maintainer-clean:
 	rm -f pg_combinebackup$(X) $(OBJS)
+	rm -rf tmp_check
 
 check:
 	$(prove_check)