Kernel: Add an x86 include check+error in x86/PageDirectory.h

Author: https://github.com/jamesmintram
Commit: 17fb2adf61
Pull-request: https://github.com/SerenityOS/serenity/pull/11141
Reviewed-by: https://github.com/bgianfo ✅
Reviewed-by: https://github.com/kleinesfilmroellchen ✅

Kernel/Memory/MemoryManager.h

@@ -21,6 +21,10 @@
 #include <Kernel/Memory/Region.h>
 #include <Kernel/Memory/VMObject.h>
 
+namespace Kernel {
+class PageDirectoryEntry;
+}
+
 namespace Kernel::Memory {
 
 constexpr bool page_round_up_would_wrap(FlatPtr x)