perf tools: The buildid usage in example eBPF program

The bpf-samples/ex.c demonstrates how to put the
buildid data into eBPF program.

Link: http://lkml.kernel.org/n/tip-dq97ddil7h3qbvphbbo8p08c@git.kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>