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