perf tools: Fix static linking with libdw from elfutils

Fix feature test for static libdw: link required dependencies.  Backends
of libebl are not statically linked thus libdl is required.

In Debian/Ubuntu libdw-dev includes libebl.a starting from 0.166-1.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/150322916720.129772.7959925864494283854.stgit@buzz


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