perf tools: Explicitly add libdl dependency

Fixes compile failure on Fedora 12.

Signed-off-by: David Ahern <dsahern@gmail.com>
Link: http://lkml.kernel.org/r/1379900700-5186-3-git-send-email-dsahern@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>