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: d66763f ("perf test trace_btf_enum: Add regression test for the BTF augmentation of enums in 'perf trace'")
Signed-off-by: James Clark <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Howard Chu <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

tools/perf/tests/shell/trace_btf_enum.sh

@@ -10,6 +10,7 @@ non_syscall="timer:hrtimer_init,timer:hrtimer_start"
 
 TESTPROG="perf test -w landlock"
 
+# shellcheck source=lib/probe.sh
 . "$(dirname $0)"/lib/probe.sh
 skip_if_no_perf_trace || exit 2