systemd: fix a test-seccomp build issue

systemd: fix a test-seccomp build issue

Fix a following compiling issue when seccomp is enabled by
PACKAGECONFIG:
| ../test-seccomp.c: In function 'test_protect_sysctl':
| ../test-seccomp.c:307:5: error: "__NR__sysctl" is not defined, evaluates to 0 [-Werror=undef]
|  307 | #if __NR__sysctl > 0
|      |     ^~~~~~~~~~~~

Reference:
https://github.com/systemd/systemd/pull/14032

Signed-off-by: Ming Liu <liu.ming50@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>