selftests/pidfd: fixes syscall number defines

selftests/pidfd: fixes syscall number defines

I had to spend some (a lot;) time to understand why pidfd_info_test
(and more) fails with my patch under qemu on my machine ;) Until I
applied the patch below.

I think it is a bad idea to do the things like

	#ifndef __NR_clone3
	#define __NR_clone3 -1
	#endif

because this can hide a problem. My working laptop runs Fedora-23 which
doesn't have __NR_clone3/etc in /usr/include/. So "make" happily succeeds,
but everything fails and it is not clear why.

Link: https://lore.kernel.org/r/20250323174518.GB834@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>