x86/head: Simplify relative include path to xen-head.S

Fix the relative path specification in the include directives adding
xen-head.S to the kernel's head_*.S files since they both have
"arch/x86/" as prefix.

  [ bp: Rewrite commit message. ]
Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231231121904.24622-1-ytcoode@gmail.com

arch/x86/kernel/head_32.S

@@ -500,7 +500,7 @@ __INITRODATA
 int_msg:
 	.asciz "Unknown interrupt or fault at: %p %p %p\n"
 
-#include "../../x86/xen/xen-head.S"
+#include "../xen/xen-head.S"
 
 /*
  * The IDT and GDT 'descriptors' are a strange 48-bit object

arch/x86/kernel/head_64.S

@@ -720,7 +720,7 @@ SYM_DATA(smpboot_control,		.long 0)
 SYM_DATA(phys_base, .quad 0x0)
 EXPORT_SYMBOL(phys_base)
 
-#include "../../x86/xen/xen-head.S"
+#include "../xen/xen-head.S"
 
 	__PAGE_ALIGNED_BSS
 SYM_DATA_START_PAGE_ALIGNED(empty_zero_page)