libtasn1: fix botched makeinfo/missing logic

The makeinfo/missing logic is botched leading to failures such as:
http://autobuild.buildroot.net/results/5f0/5f0e8f62be86b56813f5fc0510e83289ed2b0410/


Just disable documentation build the hard way.

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