mod_session_dbd: follow up to r1687021.

Move the new pool argument of dbd_load() first as the other functions in the
module (no functional change).
Suggested by: mrumph

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1687087 13f79535-47bb-0310-9956-ffa450edef68