bpf: guard BPF_NO_PRESERVE_ACCESS_INDEX in skb_pkt_end.c

This little patch is a follow-up to:
https://lore.kernel.org/bpf/[e-mail]/T/#u

The temporary workaround of passing -DBPF_NO_PRESERVE_ACCESS_INDEX
when building with GCC triggers a redefinition preprocessor error when
building progs/skb_pkt_end.c. This patch adds a guard to avoid
redefinition.

Signed-off-by: Jose E. Marchesi <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: Eduard Zingerman <[e-mail]>
Cc: Yonghong Song <[e-mail]>
Cc: Andrii Nakryiko <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/progs/skb_pkt_end.c

···
 // SPDX-License-Identifier: GPL-2.0
+#ifndef BPF_NO_PRESERVE_ACCESS_INDEX
 #define BPF_NO_PRESERVE_ACCESS_INDEX
+#endif
 #include <vmlinux.h>
 #include <bpf/bpf_core_read.h>
 #include <bpf/bpf_helpers.h>