perf unwind: Fix a compile error

perf unwind: Fix a compile error

When libunwind is on, there is a compile error as :

  util/unwind-libunwind.c:363:21: error: 'dso' undeclared (first use in this function)
      dso__data_put_fd(dso);

This patch fixes it.

Signed-off-by: Hou Pengyang <houpengyang@huawei.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 4bb11d012ab248d0 ("perf tools: Add dso__data_get/put_fd()")
Link: http://lkml.kernel.org/r/1434453395-10560-1-git-send-email-houpengyang@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>