#3 functions missing in Bento4CDll

[Mizutama]

AP4_Sample_GetDuration
AP4_Sample_SetDuration
are not described in Bento4CDll.def .

For your information , I'm trying to use your library in C#(WPF).
I'm making pure C# interop (not using C++/CLI) therefore I'm using Bento4CDll.
After I saisfied , I could give you the result.