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/bp...