This was only used for some extra assertions. It' is not essential.
If this would be for real usage, we should add a dependancy so that
nm-utils/nm-enum-utils.c requires nm-hash-utils.h. But as it is,
this is not necessary.

This fixes build for meson, which wrongly tries to build nm-enum-utils.c
for libnm-util, but then fails to include nm-hash-utils.c. That should
be fixed independently.

Fixes: 84a6eff1