package/ntp: fix build for toolchains without ssp

package/ntp: fix build for toolchains without ssp

ntp defaults to use ssp support
http://bk1.ntp.org/ntp-stable/sntp/harden/README

by using these flags
http://bk1.ntp.org/ntp-stable/sntp/harden/linux

If the toolchain lacks SSP support this patch forces ntp to use an
empty set of flags: http://bk1.ntp.org/ntp-stable/sntp/harden/default

Fixes
http://autobuild.buildroot.net/results/1d5/1d58bd8745b22c8eb71fea4c7255d3ace69f6f7a/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>