selftests/bpf: Remove duplicate include header in xdp_hw_metadata

The linux/net_tstamp.h is included more than once, thus clean it up.

Signed-off-by: Ye Xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/202301311440516312161@zte.com.cn