SystemInformation: fix conversion warning

SystemInformation.cxx: In member function 'bool kwsys::SystemInformationImplementation::QueryProcessor()':
SystemInformation.cxx:3029:31: warning: conversion to 'unsigned int' from 'long int' may alter its value [-Wconversion]

Change-Id: I622072bf176d96814a8b17ce66b099cbd797913a