selftests/bpf: Fix selftest build with old libc

pidfd_open was added in 2019. Some versions of libc library don't define it.
Define it manually if it's not available.

Reported-by: Sergei Iudin <siudin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Alexei Starovoitov 2020-11-06 12:33:46 -08:00
commit f055f355fa