Fix a compilation warning introduced by r1802040.

mod_proxy_fcgi.c:893:19: warning: ‘flushpoll’ may be used uninitialized in this function [-Wmaybe-uninitialized]

This warning is a false positive.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1805490 13f79535-47bb-0310-9956-ffa450edef68