x86/vdso: Ensure all kernel code is seen by objtool

extable.c is kernel code and not part of the VDSO

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111143.512144110@infradead.org