Add missing include guards

These are necessary for unity builds.

src/object/C4MeshDenumerator.h

@@ -15,6 +15,9 @@
  * for the above references.
  */
 
+#ifndef INC_C4MeshDenumerator
+#define INC_C4MeshDenumerator
+
 #include "lib/StdMesh.h"
 #include "object/C4ObjectPtr.h"
 
@@ -38,3 +41,5 @@ public:
 };
 
 extern const StdMeshInstance::AttachedMesh::DenumeratorFactoryFunc C4MeshDenumeratorFactory;
+
+#endif

src/platform/C4SoundInstance.h

@@ -17,6 +17,8 @@
 
 /* Helper classes for individual sounds and effects in sound system. */
 
+#ifndef INC_C4SoundInstance
+#define INC_C4SoundInstance
 #include "platform/C4SoundSystem.h"
 
 class C4Object; 
@@ -101,3 +103,4 @@ public:
 	void SetPlayer(int32_t new_player);
 };
 
+#endif