c-ares: define random file

When cross-compiling it assumes there's none, however that's not true.
Specify the usual /dev/urandom for this usage, it's used for random key
generation on startup.

Signed-off-by: Gustavo Zacarias <gustavo.zacarias@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>