selftests/seccomp: Support glibc 2.26 siginfo_t.h

The 2.26 release of glibc changed how siginfo_t is defined, and the earlier
work-around to using the kernel definition are no longer needed. The old
way needs to stay around for a while, though.

Reported-by: Seth Forshee <seth.forshee@canonical.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>