x86/xen: Add unwind hint annotations to xen_setup_gdt

Not needed in mainline as this function got rewritten in 4.12

This enables objtool to grok the iret in the middle of a C function.

This matches commit 76846bf3cb09 ("x86/asm: Add unwind hint annotations
to sync_core()")

Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>