t/lib-httpd/apache.conf: configure an MPM module for apache 2.4

Versions of Apache before 2.4 always had a "MultiProcessing Module" (MPM) statically built in, which manages the worker threads/processes. We do not care which one, as it is largely a performance issue, and we put only a light load on the server during our testing. As of Apache 2.4, the MPM module is loadable just like any other module, but exactly one such module must be loaded. On a system where the MPMs are compiled dynamically (e.g., Debian unstable), this means that our test Apache server will not start unless we provide the appropriate configuration. Unfortunately, we do not actually know which MPM modules are available or appropriate for the system on which the tests are running. This patch picks the "prefork" module, as it is likely to be available on all Unix-like systems. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>