rrdtool: fix file locations

When we build rrdtool, we would meet the following QA issue.

  QA Issue: /usr/share/rrdtool/examples/stripes.py_rrdtool-perl contained
  in package rrdtool-perl requires /usr/bin/python, but no providers found
  in its RDEPENDS [file-rdeps]

The problem is that /usr/share/rrdtool/examples not only contain perl examples,
but also contains python examples.

Besides, ${datadir}/examples don't exist, fix it to be ${datadir}/rrdtool/examples.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>