perf trace: empty fix libbpf 1.0+ compatibility

Avoid use of tools/perf/include/bpf/bpf.h and use the more regular BPF
headers. Add raw_syscalls:sys_enter to avoid the evlist being empty.

Committer testing:

# time perf trace -e ~acme/git/perf/tools/perf/examples/bpf/empty.c sleep 5

real 0m5.697s
user 0m0.217s
sys 0m0.453s
#

I.e. it sets up everything successfully (use -v to see the details) and
filters out all syscalls, then exits when the workload (sleep 5)
finishes.

Signed-off-by: Ian Rogers <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Leo Yan <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/examples/bpf/empty.c

···
-#include <bpf/bpf.h>
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
 
-license(GPL);
+struct syscall_enter_args;
+
+SEC("raw_syscalls:sys_enter")
+int sys_enter(struct syscall_enter_args *args)
+{
+	return 0;
+}
+char _license[] SEC("license") = "GPL";