nspawn: allow sched_rr_get_interval_time64 through seccomp filter

We only allow a selected subset of syscalls from nspawn containers
and don't list any time64 variants (needed for 32-bit arches when
built using TIME_BITS=64, which is relatively new).

We allow sched_rr_get_interval which cpython's test suite makes
use of, but we don't allow sched_rr_get_interval_time64.

The test failures when run in an arm32 nspawn container on an arm64 host
were as follows:
```
======================================================================
ERROR: test_sched_rr_get_interval (test.test_posix.PosixTester.test_sched_rr_get_interval)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/var/tmp/portage/dev-lang/python-3.11.0_p1/work/Python-3.11.0/Lib/test/test_posix.py", line 1180, in test_sched_rr_get_interval
    interval = posix.sched_rr_get_interval(0)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PermissionError: [Errno 1] Operation not permitted
```

Then strace showed:
```
sched_rr_get_interval_time64(0, 0xffbbd4a0) = -1 EPERM (Operation not permitted)
```

This appears to be the only time64 syscall that isn't already included one of
the sets listed in nspawn-seccomp.c that has a non-time64 variant. Checked
over each of the time64 syscalls known to systemd and verified that none
of the others had a non-time64-variant whitelisted in nspawn other than
sched_rr_get_interval.

Bug: https://bugs.gentoo.org/880131

src/nspawn/nspawn-seccomp.c

@@ -88,6 +88,7 @@ static int add_syscall_filters(
                 { 0,                  "sched_getparam"         },
                 { 0,                  "sched_getscheduler"     },
                 { 0,                  "sched_rr_get_interval"  },
+                { 0,                  "sched_rr_get_interval_time64" },
                 { 0,                  "sched_yield"            },
                 { 0,                  "seccomp"                },
                 { 0,                  "sendfile"               },