Fix compiler warnings.

../../../bench/wtperf/wtperf.c: In function ‘connection_reconfigure’:
../../../bench/wtperf/wtperf.c:797:18: error: conversion to ‘size_t’ from ‘long int’ may change the sign of the result [-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c: In function ‘config_opt_file’:
../../../bench/wtperf/wtperf.c:1213:19: error: conversion to ‘size_t’ from ‘long int’ may change the sign of the result [-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c: In function ‘config_opt_usage’:
../../../bench/wtperf/wtperf.c:1348:15: error: conversion to ‘size_t’ from ‘int’ may change the sign of the result [-Werror=sign-conversion]
../../../bench/wtperf/wtperf.c: In function ‘indent_lines’:
../../../bench/wtperf/wtperf.c:1517:16: error: conversion to ‘size_t’ from ‘long int’ may change the sign of the result [-Werror=sign-conversion]
cc1: all warnings being treated as errors