libmodplug: new package

MOD music file decoder.
This package was originally found at : https://github.com/huceke/buildroot-rbp


By gimli <ebsi4711@gmail.com>

[Peter: fix toolchain comment + file header. Add _LICENSE_FILES]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>