perf build: Make minimal shellcheck version to v0.6.0

perf build: Make minimal shellcheck version to v0.6.0

The perf build failed due to the shellcheck on my machine (v0.4.6 on Ubuntu
18.04.1 LTS) doesn't support -a/--check-sourced and -S/--severity option.

These two options are introduced in shellcheck v0.4.7 and v0.6.0
respectively. So restrict the minimal version of shellcheck to v0.6.0.

Fixes: b809fc656e763296 ("perf build: Shellcheck support for OUTPUT directory")
Reviewed-by: Ian Rogers <[email protected]>
Signed-off-by: Yicong Yang <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Junhao He <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>