x86: fix warning in "x86: clean up vSMP detection"

The function detect_vsmp_box is a void function in the PCI case.
Change the !PCI stub to void too.

Signed-off-by: Alexander van Heukelum <heukelum@fastmail.fm>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>