libct/cg/fscommon: drop go 1.13 compatibility
libct/cg/fscommon: drop go 1.13 compatibility

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

kolyshkin/runc,rhatdan/runc,opencontainers/runc,kolyshkin/runc,kolyshkin/runc,kolyshkin/runc,dqminh/runc,opencontainers/runc,dqminh/runc,rhatdan/runc,opencontainers/runc,opencontainers/runc,dqminh/runc,dqminh/runc,rhatdan/runc,rhatdan/runc