stat_item.c: Fix compiler warnings

  CC       stat_item.lo
stat_item.c: In function 'osmo_stat_item_group_alloc': stat_item.c:84:28: warning: cast to pointer from integer of different
size [-Wint-to-pointer-cast]
   group->items[item_idx] = (void *)items_size;
                            ^
stat_item.c:98:24: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
    ((uint8_t *)items + (int)group->items[item_idx]);
                        ^
1 file changed