selftests/harness: Update named initializer syntax
The harness was still using old-style GNU named initializer syntax.
Fix this so Clang will stop warning:
seccomp_bpf.c:2924:1: warning: use of GNU old-style field designator extension
[-Wgnu-designator]
./../kselftest_harness.h:147:25: note: expanded from macro 'TEST'
^
./../kselftest_harness.h:172:5: note: expanded from macro '__TEST_IMPL'
fn: &test_name, termsig: _signal }; \
^