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_vt.cpp

@@ -873,7 +873,7 @@ void daE_VA_c::setFireEffect(int param_0) {
         dComIfGp_particle_set(field_0x10f80[idx + 2], 0x3AE, &field_0x1140[param_0], NULL, &scale);
 }
 
-f32 dummy() {
+static f32 dummy() {
     return 3.2f;
 }