eal: explicit cast of strlcpy return

eal: explicit cast of strlcpy return

GCC 8.1 warns:
rte_string_fns.h: In function 'rte_strlcpy':
rte_string_fns.h:58:9:
warning: conversion to 'size_t' {aka 'long unsigned int'} from
'int' may change the sign of the result [-Wsign-conversion]
  return snprintf(dst, size, "%s", src);
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: 5364de644a4b ("eal: support strlcpy function")

Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>