x86/desc: Move boot_gdtr into .rodata

It is never written to.

This was an oversight when it was moved from asm into C.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/desc.c

@@ -89,7 +89,7 @@ seg_desc_t boot_compat_gdt[PAGE_SIZE / sizeof(seg_desc_t)] =
  * References boot_cpu_gdt_table for a short period, until the CPUs switch
  * onto their per-CPU GDTs.
  */
-struct desc_ptr boot_gdtr = {
+const struct desc_ptr boot_gdtr = {
     .limit = LAST_RESERVED_GDT_BYTE,
     .base = (unsigned long)(boot_gdt - FIRST_RESERVED_GDT_ENTRY),
 };