hostapd: use getrandom(2)

Motivation: Devices with few entropy sources on boot hang/block if many services try to
read from /dev/random.

https://w1.fi/cgit/hostap/commit/?id=89a7cdd690b48a0c56380cf4609442ed13527f44
states getrandom() is recommended, but not enabled by default since it
relies on:

* Linux kernel 3.17 (NixOS 19.09 has 4.19; master presumably later)
* glibc 2.25 (NixOS master has 2.27
  https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/libraries/glibc/common.nix#L37 )