perf tests: Fix an incorrect type in append_script()

perf tests: Fix an incorrect type in append_script()

The return value from the call to readlink() is ssize_t. However, the
return value is being assigned to an size_t variable 'len', so making
'len' an ssize_t.

./tools/perf/tests/tests-scripts.c:182:5-8: WARNING: Unsigned expression compared with zero: len < 0.

Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=11909
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241115091527.128923-1-jiapeng.chong@linux.alibaba.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>