community/openmw: fix build

Fix build with clang. Error found on the 3.21 builders (error log).

/home/buildozer/aports/community/openmw/src/openmw-openmw-0.47.0/components/vfs/filesystemarchive.cpp: In member function 'virtual void VFS::FileSystemArchive::listResources(std::map<std::__cxx11::basic_string<char>, VFS::File*>&, char (*)(char))':
/home/buildozer/aports/community/openmw/src/openmw-openmw-0.47.0/components/vfs/filesystemarchive.cpp:41:22: error: 'transform' is not a member of 'std'
   41 |                 std::transform(proper.begin() + prefix, proper.end(), std::back_inserter(searchable), normalize_function);
      |                      ^~~~~~~~~