math/openfst: unbreak on 9.x

symbol-table-ops.cc: In function 'bool fst::AddAuxiliarySymbols(const string&, int64, int64, fst::SymbolTable*)':
symbol-table-ops.cc:131:43: error: 'to_string' is not a member of 'std'

if (index != syms->AddSymbol(prefix + std::to_string(i), index)) {
                                      ^

Reported by: pkg-fallout
Approved by: portmgr blanket