rdisc: ifdef only apply on historic glibc < 2

This fix compilation on uclibc accidentally broken by
commit c9ecbd72ff6787354111a47985b1bee5d699e39d.
Tested on uClibc 0.9.33.2 with 3.4 headers.

Signed-off-by: David Heidelberger <david.heidelberger@ixit.cz>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>