udhcpc: do not test for srand init, just do it at the start

udhcpc: do not test for srand init, just do it at the start

function                                             old     new   delta
udhcpc_main                                         2855    2867     +12
static.initialized                                     1       -      -1
client_background                                     32      31      -1
perform_release                                      124     122      -2
random_xid                                            38       -     -38
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 1/2 up/down: 12/-42)            Total: -30 bytes

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