fix the calculation of request time in mod_status

(currently nobody sets the start/stop times so this fix isn't
immediately apparent)

Submitted by:    Stas Bekman
Reviewed by:     Jeff Trawick


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93901 13f79535-47bb-0310-9956-ffa450edef68