lib: test_objpool: add missing MODULE_DESCRIPTION() macro

lib: test_objpool: add missing MODULE_DESCRIPTION() macro

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/test_objpool.o

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Link: https://lkml.kernel.org/r/20240715-md-lib-test_objpool-v2-1-5a2b9369c37e@quicinc.com
Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Reviewed-by: Matt Wu <wuqiang.matt@bytedance.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>