perf test trace_btf_general: Fix shellcheck warning

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

  tests/shell/trace_btf_general.sh line 8:
  . "$(dirname $0)"/lib/probe.sh
  ^--------------------------^ SC1090: Can't follow non-constant source.
  Use a directive to specify location.

Fixes: 0255338 ("perf trace: Add tests for BTF general augmentation")
Signed-off-by: James Clark <[email protected]>
Acked-by: Namhyung Kim <[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: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: james-c-linaro

tools/perf/tests/shell/trace_btf_general.sh

@@ -5,6 +5,7 @@
 err=0
 set -e
 
+# shellcheck source=lib/probe.sh
 . "$(dirname $0)"/lib/probe.sh
 
 file1=$(mktemp /tmp/file1_XXXX)