selftests/bpf: Fix bpf selftest build warning

selftests/bpf: Fix bpf selftest build warning

On linux-next, build for bpf selftest displays a warning:

Warning: Kernel ABI header at 'tools/include/uapi/linux/if_xdp.h'
differs from latest version at 'include/uapi/linux/if_xdp.h'.

Commit 8066e388be48 ("net: add UAPI to the header guard in various network headers")
changed the header guard from _LINUX_IF_XDP_H to _UAPI_LINUX_IF_XDP_H
in include/uapi/linux/if_xdp.h.

To resolve the warning, update tools/include/uapi/linux/if_xdp.h
to align with the changes in include/uapi/linux/if_xdp.h

Fixes: 8066e388be48 ("net: add UAPI to the header guard in various network headers")
Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Closes: https://lore.kernel.org/all/c2bc466d-dff2-4d0d-a797-9af7f676c065@linux.ibm.com/
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Saket Kumar Bhaskar <skb99@linux.ibm.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://patch.msgid.link/20250527054138.1086006-1-skb99@linux.ibm.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>