selftests/seccomp: fix build on older kernels

selftests/seccomp: fix build on older kernels

The seccomp selftest goes to some length to build against older kernel
headers, viz. all the #ifdefs at the beginning of the file.

Commit 201766a20e30 ("ptrace: add PTRACE_GET_SYSCALL_INFO request")
introduces some additional macros, but doesn't do the #ifdef dance.
Let's add that dance here to avoid:

gcc -Wl,-no-as-needed -Wall  seccomp_bpf.c -lpthread -o seccomp_bpf
In file included from seccomp_bpf.c:51:
seccomp_bpf.c: In function ‘tracer_ptrace’:
seccomp_bpf.c:1787:20: error: ‘PTRACE_EVENTMSG_SYSCALL_ENTRY’ undeclared (first use in this function); did you mean ‘PTRACE_EVENT_CLONE’?
  EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../kselftest_harness.h:608:13: note: in definition of macro ‘__EXPECT’
  __typeof__(_expected) __exp = (_expected); \
             ^~~~~~~~~
seccomp_bpf.c:1787:2: note: in expansion of macro ‘EXPECT_EQ’
  EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
  ^~~~~~~~~
seccomp_bpf.c:1787:20: note: each undeclared identifier is reported only once for each function it appears in
  EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../kselftest_harness.h:608:13: note: in definition of macro ‘__EXPECT’
  __typeof__(_expected) __exp = (_expected); \
             ^~~~~~~~~
seccomp_bpf.c:1787:2: note: in expansion of macro ‘EXPECT_EQ’
  EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
  ^~~~~~~~~
seccomp_bpf.c:1788:6: error: ‘PTRACE_EVENTMSG_SYSCALL_EXIT’ undeclared (first use in this function); did you mean ‘PTRACE_EVENT_EXIT’?
    : PTRACE_EVENTMSG_SYSCALL_EXIT, msg);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
../kselftest_harness.h:608:13: note: in definition of macro ‘__EXPECT’
  __typeof__(_expected) __exp = (_expected); \
             ^~~~~~~~~
seccomp_bpf.c:1787:2: note: in expansion of macro ‘EXPECT_EQ’
  EXPECT_EQ(entry ? PTRACE_EVENTMSG_SYSCALL_ENTRY
  ^~~~~~~~~
make: *** [Makefile:12: seccomp_bpf] Error 1

[skhan@linuxfoundation.org: Fix checkpatch error in commit log]
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
Fixes: 201766a20e30 ("ptrace: add PTRACE_GET_SYSCALL_INFO request")
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>