Fix compilation with gcc.

etext is already declared in machine/cpu.h, move kernphys declaration
there too.

Based on the patch by:	bde
MFC after:	13 days

sys/amd64/amd64/pmap.c

@@ -7562,8 +7562,6 @@ pmap_pti_free_page(vm_page_t m)
 	return (true);
 }
 
-extern char kernphys[], etext[];
-
 static void
 pmap_pti_init(void)
 {

sys/amd64/include/cpu.h

@@ -70,6 +70,7 @@ struct cpu_ops {
 extern struct	cpu_ops cpu_ops;
 extern char	btext[];
 extern char	etext[];
+extern char	kernphys[];
 
 /* Resume hook for VMM. */
 extern	void (*vmm_resume_p)(void);