perf diff: Use llabs() with 64-bit values

perf diff: Use llabs() with 64-bit values

To fix this build error on a debian mipsel cross build environment:

  builtin-diff.c: In function 'compute_cycles_diff':
  builtin-diff.c:649:10: error: absolute value function 'labs' given an argument of type 's64' {aka 'long long int'} but has parameter of type 'long int' which may cause truncation of value [-Werror=absolute-value]
    649 |    val = labs(pair->block_info->cycles_spark[i] -
        |          ^~~~

Fixes: cebf7d51a6c3 ("perf diff: Report noisy for cycles diff")
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-pn7szy5uw384ntjgk6zckh6a@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>