linux: Use socket-constants-time64.h on tst-socket-timestamp-compat

The kernel header might not define the SO_TIMESTAMP{NS}_OLD or
SO_TIMESTAMP{NS}_NEW if it older than v5.1.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>