perf inject: Fix broken perf inject -b

perf inject -b was broken. It would not inject any build_id into the
stream. Furthermore, it would strip samples from the stream.

The reason was a missing initialization of the event attribute
structure. The perf_tool.tool.attr() callback was pointing to a simple
repipe. But there was no initialization of the internal data structures
to keep track of events and event ids. That later caused event id
lookups to fail, and sample would get removed.

The patch simply adds back the call to perf_event__process_attr() to
initialize the evlist structure and now build_ids are again injected.

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1337081295-10303-2-git-send-email-eranian@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>