x86: add stringify header

We use a __stringify construction at paravirt_patch_64.c.
It's better practice to include the stringify header directly

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>