Fix includes and forward declarations

engine/core/filesystem/Filesystem.h

@@ -27,13 +27,12 @@ OTHER DEALINGS IN THE SOFTWARE.
 #pragma once
 
 #include "File.h"
+#include "Vector.h"
+#include "DynamicString.h"
 
 namespace crown
 {
 
-class FileSource;
-class DynamicString;
-
 /// Provides a platform-independent way to access files and directories
 /// on the host filesystem.
 ///