selftests: seccomp: fix compile error seccomp_bpf

aarch64-linux-gnu-gcc -Wl,-no-as-needed -Wall
    -lpthread seccomp_bpf.c -o seccomp_bpf
seccomp_bpf.c: In function 'tracer_ptrace':
seccomp_bpf.c:1720:12: error: '__NR_open' undeclared
    (first use in this function)
  if (nr == __NR_open)
            ^~~~~~~~~
seccomp_bpf.c:1720:12: note: each undeclared identifier is reported
    only once for each function it appears in
In file included from seccomp_bpf.c:48:0:
seccomp_bpf.c: In function 'TRACE_syscall_ptrace_syscall_dropped':
seccomp_bpf.c:1795:39: error: '__NR_open' undeclared
    (first use in this function)
  EXPECT_SYSCALL_RETURN(EPERM, syscall(__NR_open));
                                       ^
open(2) is a legacy syscall, replaced with openat(2) since 2.6.16.
Thus new architectures in the kernel, such as arm64, don't implement
these legacy syscalls.

Fixes: a33b2d03 ("selftests/seccomp: Add tests for basic ptrace
actions")
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Cc: stable@vger.kernel.org
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>