Fix MOD_OPTIONAL_HOOK_EXPORT_H #include guard spelling (silliest compile

warning in 2.2 codebase?)
Backports: r1514215
Submitted by: trawick



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1691240 13f79535-47bb-0310-9956-ffa450edef68