debian/watch: adding watch file

The watch file will detect new version on upstream. Unfortunately Zarafa
didn't provide a cryptographic signature on the download files so we are
unable to check if the provided files are original files from Zarafa.
debian/rules: correcting install of zarafa.pc for multiarch support

The default installation of /usr/lib/pkgconfig/zarafa.pc doesn't fit
Debians multiarch support. The .pc file has to be installed into
/usr/lib/$DEB_HOST_MULTIARCH/pkconfig/ instead.