LibIPC: Move MessageBuffer forward declaration from Stub.h to Forward.h

The type of MessageBuffer will be changing, and it was a bit awkward to
look around to find where the forward declaration was. This patch just
moves it to the obvious forwarding header.

Userland/Libraries/LibIPC/Forward.h

@@ -13,6 +13,7 @@ namespace IPC {
 class Decoder;
 class Encoder;
 class Message;
+struct MessageBuffer;
 class File;
 class Stub;
 

Userland/Libraries/LibIPC/Stub.h

@@ -9,6 +9,7 @@
 
 #include <AK/ByteString.h>
 #include <AK/OwnPtr.h>
+#include <LibIPC/Forward.h>
 
 namespace AK {
 class BufferStream;
@@ -16,9 +17,6 @@ class BufferStream;
 
 namespace IPC {
 
-class Message;
-struct MessageBuffer;
-
 class Stub {
 public:
     virtual ~Stub() = default;