Crash when scanning modplug files on amd64

I get a segfault when adding a directory that contains files whose metadata is to be retrieved with the help of modplug. I'll attach a patch that fixes the issue for me by explicitly declaring the argument and result types for all used functions (I find this cleaner and safer than relying on implicit defaults, but I guess just setting the modplug.ModPlug_Load.restype might be enough because ctypes defaults to int here wich is to small to hold the returned pointer).

As for the used version: I tested this with the current trunk (revno: 2746)