add python-ethtool/netlink-address.c to MANIFEST

It's not clear to me that we need a handcoded MANIFEST file, but
if we're going to have it, it ought to be complete.

Author: davidmalcolm

MANIFEST

@@ -9,6 +9,7 @@ python-ethtool/etherinfo_ipv6_obj.c
 python-ethtool/etherinfo_struct.h
 python-ethtool/etherinfo.h
 python-ethtool/etherinfo_obj.h
+python-ethtool/netlink-address.c
 man/pethtool.8.asciidoc
 man/pifconfig.8.asciidoc
 setup.py