perf test trace_btf_enum: Fix shellcheck warning

perf test trace_btf_enum: Fix shellcheck warning

Shellcheck versions < v0.7.2 can't follow this path so add the helper to
fix the following warning:

  In tests/shell/trace_btf_enum.sh line 13:
  . "$(dirname $0)"/lib/probe.sh
  ^--------------------------^ SC1090: Can't follow non-constant source.
  Use a directive to specify location.

Fixes: d66763fed30f0bd8 ("perf test trace_btf_enum: Add regression test for the BTF augmentation of enums in 'perf trace'")
Signed-off-by: James Clark <james.clark@linaro.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Ian Rogers <irogers@google.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/20240809095426.3065163-1-james.clark@linaro.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>