[mlir][Quasipolynomial] Fixed -Wunused-variable in GeneratorFunction.h (#76419)

```
llvm-project/mlir/lib/Analysis/Presburger/GeneratingFunction.h:56:28:
error: unused variable 'term' [-Werror,-Wunused-variable]
   56 |     for (const ParamPoint &term : numerators)
      |                            ^~~~
1 error generated.
```