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>

tools/testing/selftests/bpf/prog_tests/test_local_storage.c

@@ -12,6 +12,10 @@
 #include "local_storage.skel.h"
 #include "network_helpers.h"
 
+#ifndef __NR_pidfd_open
+#define __NR_pidfd_open 434
+#endif
+
 static inline int sys_pidfd_open(pid_t pid, unsigned int flags)
 {
 	return syscall(__NR_pidfd_open, pid, flags);