Note notice.txt's dependency on version.py

This dependency has been in effect since the notice build was changed
to use the main conf.py, due to its unconditional execfile('version.py').
Adding another conditional in conf.py seems to add needless complication,
it is easier to just note the dependency in the Makefile and carry on.

https://github.com/krb5/krb5/commit/9031b739fc4f9e4215e537855a853d2134a69ba9
Author: Ben Kaduk <kaduk@mit.edu>
Commit: 9031b739fc4f9e4215e537855a853d2134a69ba9
Branch: master
 src/doc/Makefile.in |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)