Update dependency on gnat_util

[dalybrown]

To build gnatcov from source, we require gnat_util:

gnatcoverage/tools/gnatcov/README.md

Line 85 in 29f3005

4. If your GNAT toolchain does not come with a `gnat_util` library

.

However, this (as far as I know) is not distributed with the FSF version of gnat and AdaCore has stopped releasing the community edition. There is a patch for an older version of gnat_util on the alire project in github, but that seems like an unsustainable workaround. Is there a way to build gnatcov without depending on gnat_util? Thanks!