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 <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Joe Stringer <[e-mail]>
Cc: Wang Nan <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

samples/bpf/trace_output_user.c

···
 	char data[];
 };
 
-void perf_event_read(print_fn fn)
+static void perf_event_read(print_fn fn)
 {
 	__u64 data_tail = header->data_tail;
 	__u64 data_head = header->data_head;