msgunfmt: Fix msgunfmt-tcl-1 failure with tcl version 9.0 beta2.

msgunfmt: Fix msgunfmt-tcl-1 failure with tcl version 9.0 beta2.

Reported by Jeffrey Cliff <jeffrey.cliff@gmail.com> in
<https://lists.gnu.org/archive/html/bug-gettext/2024-07/msg00000.html>.

* gettext-tools/src/msgunfmt.tcl: Access the ::msgcat::header variable through
an absolute name syntax, not a relative name syntax.

Rationale: <https://core.tcl-lang.org/tcl/wiki?name=Migrating+scripts+to+Tcl+9>
says:
  "In Tcl 8 variable names that are not absolute are resolved by looking
   first in the current namespace, and then in the global namespace.
   In Tcl 9 such variables are always interpreted as relative to the current
   namespace."