Prevent make distclean removing 0 sized file

init__.py used by Python packages typically has nothing in it
including contrib/pyzfs/libzfs_core/test/init__.py, however this
causes make distclean to delete the file.

This is the only file with size 0, and it seems reasonable to have
a comment to avoid being deleted, rather than trying to modify
distclean behavior.

1. find . -size 0 ./contrib/pyzfs/libzfs_core/test/__init__.py
2. ./autogen.sh ; ./configure ; make -j8
3. make distclean
4. ls contrib/pyzfs/libzfs_core/test/__init.py ls: cannot access 'contrib/pyzfs/libzfs_core/test/init__.py': No such file or directory
5. git diff diff --git a/contrib/pyzfs/libzfs_core/test/__init.py b/contrib/pyzfs/libzfs_core/test/init__.py deleted file mode 100644

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #7505