perf tests lib waiting: Fix the shellcheck warnings about missing shebang

Running shellcheck in "lib/waiting.sh" generates below
warning:

   In ./tools/perf/tests/shell/lib/waiting.sh line 1:
   # SPDX-License-Identifier: GPL-2.0
   ^-- SC2148 (error): Tips depend on target shell and yours is unknown. Add a shebang or a 'shell' directive.

Fix this by adding shebang in the beginning of the script.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230709182800.53002-20-atrajeev@linux.vnet.ibm.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>