build-system: make "make distcheck" work

build-system: make "make distcheck" work

It failed with:
```
make[5]: Entering directory '/home/travis/build/karelzak/util-linux/util-linux-2.36.32-0fea/_build/sub/po'
make[5]: *** No rule to make target '../../../include/fuzz.h', needed by 'util-linux.pot-update'.  Stop.
make[5]: Leaving directory '/home/travis/build/karelzak/util-linux/util-linux-2.36.32-0fea/_build/sub/po'
Makefile:896: recipe for target 'update-po' failed
make[4]: *** [update-po] Error 2
```

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>