perf inject: With --convert-callchain ignore the dummy event for dwarf stacks

On hybrid systems there is generally >1 event and a dummy event.

The perf inject --convert-callchain option is failing to convert
perf.data files on such systems reporting "--convert-callchain requires
DWARF call graph."

The failing event is the dummy event that doesn't need to be set up for
samples.

As such ignore this event when checking the evsels.

Fixes: 92ea788d2af4e65a ("perf inject: Add --convert-callchain option")
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>