chrony: disable docs build

Needs makeinfo and we don't care about docs. Fixes:
http://autobuild.buildroot.net/results/fd0/fd03a86ed670c765d9f0a1ed6cc913079e030fc9/



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