fix these warnings:

mod_isapi.c:488: warning: no previous prototype for ‘GetServerVariable’
mod_isapi.c:590: warning: no previous prototype for ‘ReadClient’
mod_isapi.c:807: warning: no previous prototype for ‘WriteClient’
mod_isapi.c:863: warning: no previous prototype for ‘ServerSupportFunction’
mod_isapi.c:1407: warning: no previous prototype for ‘isapi_handler’



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@918665 13f79535-47bb-0310-9956-ffa450edef68