fix these warnings (and presumably a similar one in mod_socache_dc.c):

mod_socache_shmcb.c:625: warning: no previous prototype for ‘socache_shmcb_iterate’
mod_socache_dbm.c:505: warning: no previous prototype for ‘socache_dbm_iterate’
mod_socache_memcache.c:290: warning: no previous prototype for ‘socache_mc_iterate’


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@918655 13f79535-47bb-0310-9956-ffa450edef68