Fix compiler warning in wtperf:

../../../bench/wtperf/wtperf.c:789:27: error: conversion to 'long long
unsigned int' from 'long int' may change the sign of the result
[-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c:789:13: error: conversion to 'long long
unsigned int' from 'time_t' may change the sign of the result
[-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c:790:30: error: conversion to 'long long
unsigned int' from 'long int' may change the sign of the result
[-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c:790:15: error: conversion to 'long long
unsigned int' from 'time_t' may change the sign of the result
[-Werror=sign-conversion]