mpc: fix the extinfo initial value

extinfo means the number of data fields

And on cygwin, we have :

   $ cat /proc/stat
   cpu 11470140 0 9621000 1577205859
   cpu0 11470140 0 9621000 1577205859
   page 21707753 5271884
   swap 21707753 5106278
   intr 147103242
   ctxt 1150146723
   btime 1360807383