rrdtool: add patch to disable documentation

Add patch to disable documentation. Fixes:
http://autobuild.buildroot.net/results/6fb/6fb94127af6ff53bdcf52ee0eec8971ccd8858f5/

Patch status: pull req https://github.com/oetiker/rrdtool-1.x/pull/690



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>