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 <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

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

···
    Copyright (C) 2018 Red Hat, Inc., Arnaldo Carvalho de Melo <[e-mail]>
 */
 
-#include <bpf/bpf.h>
+#include <bpf.h>
 
 #define NSEC_PER_SEC	1000000000L