perf tools: Remove unneccessary feature-dwarf warning

Don't warn for feature-dwarf==0 if user explicitily disabled DWARF by
using NO_DWARF=1.

Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20170112210159.76143-1-davidcc@google.com


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