eal: fix casts in random functions

eal: fix casts in random functions

GCC 8.1 warns:

In function 'rte_srand':
rte_random.h:34:10:
warning: conversion to 'long int' from 'long unsigned int'
may change the sign of the result [-Wsign-conversion]
  srand48((long unsigned int)seedval);

rte_random.h:51:8:
warning: conversion to 'uint64_t' {aka 'long unsigned int'}
from 'long int' may change the sign of the result
[-Wsign-conversion]
  val = lrand48();
        ^~~~~~~

rte_random.h:53:6:
warning: conversion to 'long unsigned int' from 'long int'
may change the sign of the result [-Wsign-conversion]
  val += lrand48();

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>