util: constant -1 with expression of type char

This patch resolve following warning.

  tools/perf/util/evsel.c:1620:9: error: result of comparison of constant
   -1 with expression of type 'char' is always false
   -Werror,-Wtautological-constant-out-of-range-compare
   1620 |                 if (c == -1)
        |                     ~ ^  ~~

Signed-off-by: Yunseong Kim <yskelg@gmail.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Austin Kim <austindh.kim@gmail.com>
Cc: shjy180909@gmail.com
Cc: Ze Gao <zegao2021@gmail.com>
Cc: Leo Yan <leo.yan@linux.dev>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240619203428.6330-2-yskelg@gmail.com