samples/bpf: Make perf_event_read() static

samples/bpf: Make perf_event_read() static

While testing Joe's conversion of samples/bpf/ to use tools/lib/bpf/ I noticed
some warnings building samples/bpf/ on a Fedora Rawhide container, with
clang/llvm 3.9 I noticed this:

  [root@1e797fdfbf4f linux]# make -j4 O=/tmp/build/linux/ samples/bpf/
  make[1]: Entering directory '/tmp/build/linux'
    CHK     include/config/kernel.release
    GEN     ./Makefile
    CHK     include/generated/uapi/linux/version.h
    Using /git/linux as source for kernel
  <SNIP>
    HOSTCC  samples/bpf/trace_output_user.o
  /git/linux/samples/bpf/trace_output_user.c:64:6: warning: no previous
  prototype for 'perf_event_read' [-Wmissing-prototypes]
   void perf_event_read(print_fn fn)
        ^~~~~~~~~~~~~~~
    HOSTLD  samples/bpf/trace_output
  make[1]: Leaving directory '/tmp/build/linux'

Shut up the compiler by making that function static.

Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Joe Stringer <joe@ovn.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20161215152927.GC6866@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>