In musl, `stdout` is a macro that expands to `(stdout)`, and
`::(stdout)` is not valid C++.
../mu-1.12.2/lib/utils/mu-utils.hh:268:32: error: expected
id-expression before '(' token
268 | ::stdout);
| ^~~~~~
Nothing in the Mu namespace is named stdout, so it is safe to drop
the `::`.
Bug: https://bugs.gentoo.org/928361
45b4f60348