FEXCore: Moves CPUBackend definition internal

This is no longer necessary to be part of the public API. Moves the
header internally.

Needed to pass through `IsAddressInCodeBuffer` from CPUBackend through
the Context object, but otherwise no functional change.

FEXCore/include/FEXCore/Debug/InternalThreadState.h

@@ -2,7 +2,6 @@
 #pragma once
 #include <FEXCore/Core/Context.h>
 #include <FEXCore/Core/CoreState.h>
-#include <FEXCore/Core/CPUBackend.h>
 #include <FEXCore/Core/SignalDelegator.h>
 #include <FEXCore/IR/IntrusiveIRList.h>
 #include <FEXCore/IR/RegisterAllocationData.h>
@@ -28,6 +27,7 @@ namespace FEXCore::Context {
 }
 
 namespace FEXCore::CPU {
+  class CPUBackend;
   union Relocation;
 }