Follow-up to r1523465:  Fix compiler warning.

* subversion/svnserve/svnserve.c
  (acquire_connection_pool): empty parameter lists must be 'void'

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1523469 13f79535-47bb-0310-9956-ffa450edef68