perf tools powerpc: Fix build issue when DWARF support is disabled

The powerpc skip callchain code uses DWARF, so we must disable it if
DWARF is disabled.

Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20140825182506.2be6512d@kryten
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>