x86/mm: Remove unnecessary include of <linux/extable.h>

The header file linux/extable.h is included for
search_exception_tables(). That function is no longer used since commit:

  c2508ec5 ("mm: introduce new 'lock_mm_and_find_vma()' page fault helper")

Remove it.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20241220084029.473617-1-bigeasy@linutronix.de