Mark the arm64 machdep.h as kernel only

None of this is useful for userspace.

Sponsored by:	Innovate UK

sys/arm64/include/machdep.h

@@ -29,6 +29,8 @@
 #ifndef _MACHINE_MACHDEP_H_
 #define	_MACHINE_MACHDEP_H_
 
+#ifdef _KERNEL
+
 struct arm64_bootparams {
 	vm_offset_t	modulep;
 	vm_offset_t	kern_l1pt;	/* L1 page table for the kernel */
@@ -56,4 +58,6 @@ void parse_fdt_bootargs(void);
 #endif
 extern void (*pagezero)(void *);
 
+#endif /* _KERNEL */
+
 #endif /* _MACHINE_MACHDEP_H_ */