userlandfs: Pre-define and use the right KMessage.

Otherwise, later on when we include the real one Clang errors about
there being two conflicting KMessages declared in separate contexts.

src/add-ons/kernel/file_systems/userlandfs/kernel_add_on/FileSystem.h

@@ -22,7 +22,8 @@
 
 
 struct IOCtlInfo;
-class KMessage;
+namespace BPrivate { class KMessage; }
+using BPrivate::KMessage;
 class Settings;
 class Volume;