c: Fix sizeof(dtype) parsing

sizeof was treated as unary op, and then (dtype) was treated as
cast by the recursive cExpression(). That's wrong, since it's the
sizeof operand, not a cast.

This could cause silent mistranslation in cases like:
	sizeof (int) * 2
was translated as
	sizeof(cast(int, deref(2)))
instead of
	mulbop(sizeof(int), 2)