workqueue: remove unneeded kallsyms include

The filw was converted from print_symbol() to %pf some time
ago (044c782c

 "workqueue: fix checkpatch issues").
kallsyms does not seem to be needed anymore.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>