lib: test_objpool: make global variables static

lib: test_objpool: make global variables static

Kernel test robot reported build warnings that structures g_ot_sync_ops,
g_ot_async_ops and g_testcases should be static. These definitions are
only used in test_objpool.c, so make them static

Link: https://lore.kernel.org/all/20231108012248.313574-1-wuqiang.matt@bytedance.com/
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202311071229.WGrWUjM1-lkp@intel.com/

Signed-off-by: wuqiang.matt <wuqiang.matt@bytedance.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>