mu-utils: Fix build with musl

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

lib/utils/mu-utils.hh

@@ -265,7 +265,7 @@ bool fputs_encoded (const std::string& str, FILE *stream);
 template<typename...T>
 static inline bool mu_print_encoded(fmt::format_string<T...> frm, T&&... args) noexcept {
 	return fputs_encoded(fmt::format(frm, std::forward<T>(args)...),
-			     ::stdout);
+			     stdout);
 }
 
 /**