Kernel: Fix various forward declarations

I decided to modify MappedROM.h because all other entried in Forward.h
are also classes, and this is visually more pleasing.

Other than that, it just doesn't make any difference which way we resolve
the conflicts.

Kernel/FileSystem/VirtualFileSystem.h

@@ -43,7 +43,7 @@ namespace Kernel {
 class Custody;
 class Device;
 class FileDescription;
-class UnveiledPath;
+struct UnveiledPath;
 
 struct UidAndGid {
     uid_t uid;

Kernel/Thread.h

@@ -530,7 +530,7 @@ private:
     IntrusiveListNode m_wait_queue_node;
 
 private:
-    friend class SchedulerData;
+    friend struct SchedulerData;
     friend class WaitQueue;
     bool unlock_process_if_locked();
     void relock_process(bool did_unlock);

Kernel/VM/MappedROM.h

@@ -31,7 +31,8 @@
 
 namespace Kernel {
 
-struct MappedROM {
+class MappedROM {
+public:
     const u8* base() const { return region->vaddr().offset(offset).as_ptr(); }
     const u8* end() const { return base() + size; }
     OwnPtr<Region> region;

Libraries/LibDebug/Dwarf/Expression.h

@@ -29,7 +29,7 @@
 #include "AK/ByteBuffer.h"
 #include "AK/Types.h"
 
-class PtraceRegisters;
+struct PtraceRegisters;
 
 namespace Debug::Dwarf::Expression {