evaluate: don't eval unary arguments

evaluate: don't eval unary arguments

When a unary expression is inserted to implement a byte-order
conversion, the expression being converted has already been evaluated
and so `expr_evaluate_unary` doesn't need to do so.

This is required by {ct|meta} statements with bitwise operations, which
might result in byteorder conversion of the expression.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>