perf test: Fix a memory leak in attr test

perf test: Fix a memory leak in attr test

The get_argv_exec_path() returns a dynamic memory so it should be
freed after use.

  $ perf test -v 17
  ...
  ==141682==ERROR: LeakSanitizer: detected memory leaks

  Direct leak of 33 byte(s) in 1 object(s) allocated from:
    #0 0x7f09107d2e8f in __interceptor_malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:145
    #1 0x7f091035f6a7 in __vasprintf_internal libio/vasprintf.c:71

  SUMMARY: AddressSanitizer: 33 byte(s) leaked in 1 allocation(s).

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lore.kernel.org/r/20210301140409.184570-3-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>