selftests/seccomp: Support glibc 2.26 siginfo_t.h

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 <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Will Drewry <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Tested-by: Seth Forshee <[email protected]>
Signed-off-by: Shuah Khan <[email protected]>