profiles/features/musl/package.mask: mask dev-libs/elfutils

profiles/features/musl/package.mask: mask dev-libs/elfutils

I'm specifically masking elfutils because it's a widespread
dependency that pops up all the time when I try to build ::gentoo
packages against musl profiles.

In ::gentoo dev-libs/elfutils needs an upstream port to
musl: #602126, #701478
- https://sourceware.org/PR21002
- https://sourceware.org/PR21008
- https://sourceware.org/PR21010

Overlays that carry out-of-tree patches can remove the mask
via `profiles/package.mask`:

```
    # unmask in ::${overlay}
    -dev-libs/elfutils
```

Bug: https://bugs.gentoo.org/602126
Closes: https://bugs.gentoo.org/701478
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>