hostapd: use getrandom syscall

hostapd: use getrandom syscall

hostapd will not use the getrandom() syscall and as a fallback use
/dev/random, the syscall is supported since Linux 3.17 and in the musl,
glibc and uclibc version used by OpenWrt.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>