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

Kernel/Arch/x86/MSR.h

@@ -10,6 +10,9 @@
 
 #include <Kernel/Arch/x86/CPUID.h>
 
+#include <AK/Platform.h>
+VALIDATE_IS_X86()
+
 namespace Kernel {
 
 class MSR {