Collectd.pm: Fixed the check for complete function names in plugin_register.

Collectd.pm: Fixed the check for complete function names in plugin_register.

plugin_register() checks if the registered function includes the plugin's
package name. Up to now, this check would also have succeeded if the
function name itself would be prepended by the package name. Now, the
check looks for the separator "::".

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>