Fix warning

[saleyn]

With listcomp parse transform, when compiling the following code:

[{Idx,I} || Idx, {req, FF} <- [{req, [10,20,30]}], I <- FF]

it produces the warning: Warning: variable 'FF' is unused.

This is because the code gets transformed into:

element(1,
            lists:mapfoldl(fun ({_I@5_54 = {req, FF}, I}, Idx) ->
                                   {{Idx, I}, Idx + 1}
                           end,
                           1,
                           [{_I@5_54, I}
                            || _I@5_54 = {req, FF} <- [{req, [10, 20, 30]}],
                               I <- FF]))

where the FF variable in the lambda is unused in its body. The solution is to check the body of the lambda, and prefix all unused variables.

The workaround is to prefix the unused variables with _:

[{Idx,I} || Idx, {req, _FF} <- [{req, [10,20,30]}], I <- _FF]