multimedia/mkvtoolnix: fix build with libc++ 21

With libc++ 21 multimedia/mkvtoolnix fails to build, with errors similar
to:

In file included from lib/fmt/src/os.cc:13:
In file included from lib/fmt/include/fmt/os.h:11:
lib/fmt/include/fmt/format.h:745:28: fatal error: use of undeclared identifier 'malloc'
  745 |     T* p = static_cast<T*>(malloc(n * sizeof(T)));
      |                            ^~~~~~

This is because malloc and free are defined in <cstdlib>, and os.h does
not explicitly include it. (Before libc++ 21, the fmt library was
"lucky" that <cstdlib> was included transitively, but this is no longer
the case.)

PR: 293182
Approved by: riggs (maintainer)
MFH: 2026Q1