Upgrade net/nmsg to version 0.15.1:

• Fix output stats collection.
• Plugins now found by default at $libdir/nmsg instead of lib/nmsg. This fixes problem where plugins were not found with recent Debian packages built with --libdir configure option (used to install libraries in architecture specific directory). And ./configure --with-plugindir renamed to --with-pluginsdir.
• pkgconfig also depend on libprotobuf-c headers (for third-party uses of libnmsg with pkgconfig).

Sponsored by: Farsight Security, Inc.