[libc] NFC: Fix build on systems that don't have linux/time_types.h

This broke some bots that don't have linux/time_types.h available
(libc-x86_64-debian-*).

The header is needed because of __kernel_timespec, and since this is
only needed when SYS_sched_rr_get_interval_time64 is available, guarding
the include should fix the broken bot.