Install nwfilter xml files from source directory.

Install nwfilter xml files from source directory.

During an out-of-tree build, the current working directory is the build
directory. Since the FILTERS are static and not modified or
auto-generated during the build process, they need to be explicitly
fetched from the source directory during install.

Prefix the files with $(srcdir), which gets expanded to the absolute or
relative path to the source directory, even when duing out-of-tree
builds.

Signed-off-by: Philipp Hahn <hahn@univention.de>