QPID-4889: Only installs the Swig descriptors in /usr/include

Previously the files were being installed under both /usr/include
and also /usr/share/doc/qpid. Files are now installed only once.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1486148 13f79535-47bb-0310-9956-ffa450edef68