If by the include dir you mean the config dir, it is probably somewhere in /etc (/etc/httpd or /etc/apache) if it is not there you can probably find it by examening the startup script for apache.
The startup script should be somewhere in /etc/init.d or /etc/rc.d by examining the directives used to start apache you should be able to find the config dir.
If this does not work you kan check your entire filesystem using either locate or find.
find / -type f -name "*.conf" (this uses regex to find something, sometimes grep is easier)
find / -type f | grep "apache.conf"
find / -type f | grep ".conf"
Be carefull with find if you are using a production system, it generates A LOT of disk activity...