Fix for "conflicting types for 'ldap_init'"

When compiling git master on Mac OS X, I had problems with symbol conflicts in the LDAP plugin:

      CC       openldap_la-openldap.lo
    openldap.c:670: error: conflicting types for 'ldap_init'
    /usr/include/ldap.h:1517: error: previous declaration of 'ldap_init' was here

My solution was to prefix all the collectd specific functions and datatypes with the letter 'c', as had been done in other plugins.