netsniff-ng: Build and package mausezahn

mausezahn is a multicast traffic generator which is part of the
netsniff-ng sources. This utility is needed for the upcoming
kernel-selftests-net-forwarding package. Add a new package for it.

netsniff-ng will automatically detect all installed dependencies and
build only the utilities whose dependencies are installed (meaning:
mausezahn is not build when for example libcli is not installed and
other tools are not build if for example zlib is missing). Depending
on the selected packages (netsniff-ng or mausezahn) the OpenWrt build
system has to trigger netsniff-ng's configure script, which will then
pick up and automatically build the programs (mausezahn, netsniff-ng,
trafgen, ...) for which all dependencies are installed.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>