configure: add openat2.h for open_how and RESOLVE_* flags
configure: add openat2.h for open_how and RESOLVE_* flags

struct open_how and friends reside in the openat2.h header, so include
that for the test case.

Fixes: https://github.com/axboe/liburing/issues/360
Signed-off-by: Jens Axboe <cd8c6775e60d6f67a6984377324e5290df3d5358@kernel.dk>

axboe/liburing,axboe/liburing,axboe/liburing