test/cbpf_filter: fix build when openat2.h is not available

cbpf_filter.c included <linux/openat2.h> for struct open_how and
RESOLVE_IN_ROOT, so the test failed to build on systems without that
header (e.g. older distros). liburing's compat already provides
struct open_how; add a fallback #define for RESOLVE_IN_ROOT (0x10)
when the header is missing, so the test both builds and runs on
old headers with a current kernel.

Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Yang Xiuwei <yangxiuwei@kylinos.cn>
Link: https://patch.msgid.link/20260316013621.115939-3-yangxiuwei@kylinos.cn
Signed-off-by: Jens Axboe <axboe@kernel.dk>