proc: remove unused argument in proc_pid_lookup()

[adobriyan@gmail.com: delete "extern" from prototype]
Link: http://lkml.kernel.org/r/20190114195635.GA9372@avx2


Signed-off-by: Zhikang Zhang <zhangzhikang1@huawei.com>
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>