polkit: don't build documentation and examples

Building the manpages of polkit fails with issues in xsltproc. Since
we are generally not that much interested in documentation, we disable
them. This solves the build issues like
http://autobuild.buildroot.org/results/751b71a594fa04924c7c3bb8f2c460ce1e44aa90/build-end.log

.

While we're at it, we also disable the build of the polkit example
programs.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>