x86/cpu: Add a missing prototype for arch_smt_update()

.. in order to fix a -Wmissing-prototype warning.

No functional change.

Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200109121723.8151-1-b.thiel@posteo.de