UnaryOperation: -variable and +variable doesn't make variable an lvalue

[SheldonHolmgren]

Small bugfix for UnaryOperation. Until now in code like this:

function some_view() view returns (int) {
    return -someParameter;
}

Slither believes that someParameter is an lvalue. One of the results is that slither.visitors.expression.write_var believes that someParameter is written to.

I'm happy to add some test if needed.

[0xalpharush]

Thanks @SheldonHolmgren!