fix cmake windows build of proxy lbmethod mods

fix cmake windows build of proxy lbmethod mods

(for me at least, new-to-cmake on vstudio 2022)

Creating library mod_lbmethod_bybusyness.lib and object mod_lbmethod_bybusyness.exp
mod_lbmethod_bybusyness.c.obj : error LNK2019: unresolved external symbol __imp_ap_proxy_get_busy_count referenced in function is_best_bybusyness
mod_lbmethod_bybusyness.c.obj : error LNK2019: unresolved external symbol __imp_ap_proxy_set_busy_count referenced in function is_best_bybusyness
mod_lbmethod_bybusyness.so : fatal error LNK1120: 2 unresolved externals

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1918305 13f79535-47bb-0310-9956-ffa450edef68