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/20240507095011.15867-1-jose.marchesi@oracle.com/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 <jose.marchesi@oracle.com>
Cc: david.faust@oracle.com
Cc: cupertino.miranda@oracle.com
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Yonghong Song <yonghong.song@linux.dev>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240508110332.17332-1-jose.marchesi@oracle.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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>