Prevent `make distclean` removing 0 sized file

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.

 # find . -size 0
 ./contrib/pyzfs/libzfs_core/test/__init__.py
 # ./autogen.sh ; ./configure ; make -j8
 # make distclean
 # ls contrib/pyzfs/libzfs_core/test/__init__.py
 ls: cannot access 'contrib/pyzfs/libzfs_core/test/__init__.py':
     No such file or directory
 # 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