ps: stop using AT_CLKTCK, there are more standard ways

ps: stop using AT_CLKTCK, there are more standard ways

function                                             old     new   delta
ps_main                                              537     558     +21
func_time                                             66      59      -7
get_kernel_HZ                                        102       -    -102
------------------------------------------------------------------------------
(add/remove: 0/1 grow/shrink: 1/1 up/down: 21/-109)           Total: -88 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>