package/collectd: add 'mdevents' option

The mdevents plugin is new since 5.12.0 and enabled by default in
collectd.

Add an option in Buildroot, disabled by default.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>