perf session: Fix undeclared 'oe'

Using linux-3.18.y branch, perf build fails with the following:

    $ make -s -j16 -C tools/perf V=1 WERROR=0 NO_LIBUNWIND=1 HAVE_CPLUS_DEMANGLE=1 NO_GTK2=1 NO_LIBNUMA=1 NO_STRLCPY=1 NO_BIONIC=1 prefix=/usr DESTDIR=/tmp/builddir/build all
    [...]
    util/session.c: In function ‘__perf_session__process_pipe_events’:
    util/session.c:1093:36: error: ‘oe’ undeclared (first use in this function)
      ordered_events__set_copy_on_queue(oe, true);
                                        ^
    util/session.c:1093:36: note: each undeclared identifier is reported only once for each function it appears in

This patch fixes it for linux-3.18.y branch.

Fixes: 95b33b99 ("perf inject: Copy events when reordering events in pipe mode")
Cc: <stable@vger.kernel.org> # 3.18.x
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: He Kuang <hekuang@huawei.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Paul Turner <pjt@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <alexander.levin@microsoft.com>
Cc: Simon Que <sque@chromium.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>