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

tools/testing/selftests/bpf/xdp_hw_metadata.c

@@ -24,7 +24,6 @@
 #include <linux/net_tstamp.h>
 #include <linux/udp.h>
 #include <linux/sockios.h>
-#include <linux/net_tstamp.h>
 #include <sys/mman.h>
 #include <net/if.h>
 #include <poll.h>