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 <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20221103045437.163510-6-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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";