#412 WIN32: MSVC: fix unsupported GNU extension (empty structures)

[Carlo Bramini]

MSVC is unsable to build src/sid/sid.c because sound_s is declared without items.
I read that this is a GNU extension that allows you to declare a special structure with size equal to zero.
This is unsupported on MSVC, so I did a tiny patch for fixing it in my project.

[gpz]

would you add a comment that explains why it was done like this, please?

[gpz]

applied in r45873