community/openfst: fix build with gcc 14
Fix rebuild error with gcc 14.
In file included from ./../include/fst/cache.h:35,
from ./../include/fst/arc-map.h:34,
from ./../include/fst/encode.h:36,
from encode.cc:20:
./../include/fst/fst.h: In member function 'fst::internal::FstImpl<Arc>& fst::internal::FstImpl<Arc>::operator=(const fst::internal::Fst
Impl<Arc>&)':
./../include/fst/fst.h:690:59: error: no match for 'operator=' (operand types are 'std::unique_ptr<fst::SymbolTable, std::default_delete
<fst::SymbolTable> >' and 'fst::SymbolTable*')
690 | isymbols_ = impl.isymbols_ ? impl.isymbols_->Copy() : nullptr;
| ^~~~~~~