package/collectd: add 'synproxy' option

The synproxy plugin exists since 5.8.0 and is 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>