bison: fix docs build breakage

Building docs running the binary itself to see if docs need to be
changed is somewhat skippy. Disable it. Fixes:
http://autobuild.buildroot.net/results/e8a/e8a960f38e66f31355af733e510002d45052b541/



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