Declare C4AulBCC as class instead of struct

The C++ standard says that it doesn't matter whether you forward declare
something as struct or class and then define it using the other keyword.
Unfortunately MSVC adds those keywords to its name mangling scheme,
which breaks things.

src/C4Prototypes.h

@@ -23,7 +23,7 @@
 // class declarations
 class C4AbstractApp;
 class C4Action;
-struct C4AulBCC;
+class C4AulBCC;
 class C4AulDefFunc;
 class C4AulExec;
 class C4AulFunc;