include it in "pinfo_stats_tree.c" (which defines it) and
"stats_tree_plugin.c" (which refers to it).
Make all the other routines defined in "pinfo_stats_tree.c" static, as
they're not used outside "pinfo_stats_tree.c".
Get rid of declaration of unused "register_http_stat_trees()".
svn path=/trunk/; revision=15035