Fix ActiveFunc calling convention

src/main.zig

@@ -98,7 +98,7 @@ const Buffer = struct {
     }
 };
 
-fn handleActive(win: mfb.Window(State), isActive: bool) void {
+fn handleActive(win: mfb.Window(State), isActive: bool) callconv(.C) void {
     // win.getUserData().?.*.active = isActive;
     var data = win.getUserData();
     std.log.info("got data", .{});