QPID-5273: Fixed installing the qmfgen Python packages.

The install directive was being told to intall the qmfgen direction into
a directory also named qmfgen, which is why the name was doubled.
Telling Cmake to install qmfgen into the Python site-package directory
was the correct approach.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1536876 13f79535-47bb-0310-9956-ffa450edef68