Fix MSVC compiler and linker errors

- Fix struct/class forward declaration mismatches (JAIAudience, JASTrack) causing different MSVC mangled names and unresolved symbols
- Add jsystem_stubs.cpp with stubs for JASHeap, JASVoiceBank, J3DShapeTable, JAUSection, JHICommBuf, HIO/HIO2, JOR

src/d/actor/d_a_e_fb.cpp

@@ -127,7 +127,7 @@ static f32 dummy_117095() {
 }
 #endif
 
-bool hio_set;
+static bool hio_set;
 
 static daE_FB_HIO_c l_HIO;