perf bpf examples: Fix bpf.h header include directive in 5sec.c example

It was looking at bpf/bpf.h, which caused this problem:

  # perf trace -e tools/perf/examples/bpf/5sec.c
  /home/acme/git/perf/tools/perf/examples/bpf/5sec.c:42:10: fatal error: 'bpf/bpf.h' file not found
  #include <bpf/bpf.h>
           ^~~~~~~~~~~
  1 error generated.
  ERROR:	unable to compile tools/perf/examples/bpf/5sec.c
  Hint:	Check error message shown above.
  Hint:	You can also pre-compile it into .o using:
       		clang -target bpf -O2 -c tools/perf/examples/bpf/5sec.c
       	with proper -I and -D options.
  event syntax error: 'tools/perf/examples/bpf/5sec.c'
                       \___ Failed to load tools/perf/examples/bpf/5sec.c from source: Error when compiling BPF scriptlet
  #

Change that to plain bpf.h, to make it work again:

  # perf trace -e tools/perf/examples/bpf/5sec.c sleep 5s
       0.000 perf_bpf_probe:hrtimer_nanosleep(__probe_ip: -1776891872, rqtp: 5000000000)
  # perf trace -e tools/perf/examples/bpf/5sec.c/max-stack=16/ sleep 5s
       0.000 perf_bpf_probe:hrtimer_nanosleep(__probe_ip: -1776891872, rqtp: 5000000000)
                                         hrtimer_nanosleep ([kernel.kallsyms])
                                         common_nsleep ([kernel.kallsyms])
                                         __x64_sys_clock_nanosleep ([kernel.kallsyms])
                                         do_syscall_64 ([kernel.kallsyms])
                                         entry_SYSCALL_64_after_hwframe ([kernel.kallsyms])
                                         __clock_nanosleep_2 (/usr/lib64/libc-2.32.so)
  # perf trace -e tools/perf/examples/bpf/5sec.c sleep 4s
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/examples/bpf/5sec.c

@@ -39,7 +39,7 @@
    Copyright (C) 2018 Red Hat, Inc., Arnaldo Carvalho de Melo <acme@redhat.com>
 */
 
-#include <bpf/bpf.h>
+#include <bpf.h>
 
 #define NSEC_PER_SEC	1000000000L