include: define pidfd syscalls if needed

include: define pidfd syscalls if needed

If the kernel headers are too old to provide the pidfd syscall numbers,
let's define them ourselves. This can be helpful while cross-compiling.
The runtime environment may provide a kernel that is new enough to
handle the calls, even if the toolchain doesn't.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>