Require more recent glibc on F27
    
    On CPUs with AVX-512 instruction set, ntpd sometimes segfaults because
    PTHREAD_STACK_MIN is too small. The bug has been fixed in
    glibc-2.26-24.fc27.x86_64 or later.
    
    Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1564527
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexey Slaykovsky <alexey@slaykovsky.com>
    Reviewed-By: Alexey Slaykovsky <alexey@slaykovsky.com>