buildroot

liboping: no need to check for clock_gettime

Buildroot no longer supports toolchains with glibc older than 2.17, so there
is no need to check whether librt is required for clock_* system calls.

Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>