ggml : remove dirty flag from version string (ggml/1391)

ggml : remove dirty flag from version string (ggml/1391)

This commit removes the "-dirty" suffix from the GGML version string.

The motivation for this change is to ensure that the version string
works with different ways of checking out ggml and using it in projects.
By removing the dirty flag from the version string, we avoid potential
artifacts like shared libraries getting a -dirty suffix in their names.

Instead, if the project is built from a dirty git state, the dirty flag
will be appended to the commit hash in the GGML_BUILD_COMMIT variable.
This will enable users to still identify that the build was made from
from a modified/dirty state even though the version might match a "real"
version.

For example, the commit can be produces as follows:
```c++
    printf("commit: %s\n", ggml_commit());
```
Which would print the following for a dirty build:
```console
commit: 781baf2a-dirty
```

Refs: https://github.com/ggml-org/ggml/pull/1363#issuecomment-3569691546