Fix compilation problem when __NR_preadv2 __NR_pwritev2 are undefined

Fix compilation problem when __NR_preadv2 __NR_pwritev2 are undefined

check_preadv2_pwritev2.c: In function ‘main’:
check_preadv2_pwritev2.c:12:12: error: ‘__NR_preadv2’ undeclared (first use in this function)
    syscall(__NR_preadv2, 0, NULL, 0, 0, 0);
            ^
check_preadv2_pwritev2.c:12:12: note: each undeclared identifier is reported only once for each function it appears in
check_preadv2_pwritev2.c:15:12: error: ‘__NR_pwritev2’ undeclared (first use in this function)
    syscall(__NR_pwritev2, 0, NULL, 0, 0, 0);