Fix incorrect requirement for extraParens in stringification of UOP.

[dpvc]

This fixes the parenthesis problem reported by Mike. To test, use

Context("Numeric");
$f = Compute("(-1)^x");
BEGIN_TEXT
\(\{$f->TeX\}\) = \{$f->ans_rule(20)\}
END_TEXT
ANS($f->cmp);

and check the correct answer to see if it has parentheses or not. With the patch, the parentheses should be there; without the patch, the parens will be missing.

[goehle]

Works as advertised. Thanks for the quick turnaround.