Everywhere: Forward declare structs as structs

While structs being forward declared as classes is not strictly an
issue, Clang complains as this is not portable code, since some ABIs
treat classes declared as `class` and `struct` differently.

It's easier to fix these than to reason about explicitly disabling
another warning.

Userland/DevTools/Profiler/TimelineHeader.h

@@ -11,7 +11,7 @@
 namespace Profiler {
 
 class Profile;
-class Process;
+struct Process;
 
 class TimelineHeader final : public GUI::Frame {
     C_OBJECT(TimelineHeader);

Userland/DevTools/Profiler/TimelineTrack.h

@@ -11,7 +11,7 @@
 
 namespace Profiler {
 
-class Process;
+struct Process;
 class Profile;
 class TimelineView;
 

Userland/Libraries/LibIPC/Stub.h

@@ -16,7 +16,7 @@ class BufferStream;
 namespace IPC {
 
 class Message;
-class MessageBuffer;
+struct MessageBuffer;
 
 class Stub {
 public:

Userland/Libraries/LibWeb/Forward.h

@@ -60,7 +60,7 @@ namespace Web::HTML {
 class CanvasRenderingContext2D;
 class CloseEvent;
 class DOMParser;
-class EventHandler;
+struct EventHandler;
 class HTMLAnchorElement;
 class HTMLAreaElement;
 class HTMLAudioElement;

Userland/Services/WindowServer/Compositor.h

@@ -212,7 +212,7 @@ private:
             return iterate_flush_rects(m_flush_transparent_rects);
         }
     };
-    friend class ScreenData;
+    friend struct ScreenData;
     Vector<ScreenData, default_screen_count> m_screen_data;
 
     IntrusiveList<Overlay, RawPtr<Overlay>, &Overlay::m_list_node> m_overlay_list;